diff --git a/lib/mv/membership/import/header_mapper.ex b/lib/mv/membership/import/header_mapper.ex
new file mode 100644
index 0000000..4e4a77d
--- /dev/null
+++ b/lib/mv/membership/import/header_mapper.ex
@@ -0,0 +1,396 @@
+defmodule Mv.Membership.Import.HeaderMapper do
+  @moduledoc """
+  Maps CSV headers to canonical member fields and custom fields.
+
+  Provides header normalization and mapping functionality for CSV imports.
+  Handles bilingual header variants (English/German) and custom field detection.
+
+  ## Header Normalization
+
+  Headers are normalized using the following rules:
+  - Trim whitespace
+  - Convert to lowercase
+  - Unicode normalization (ß → ss, ä → ae, ö → oe, ü → ue)
+  - Remove all whitespace (ensures "first name" == "firstname")
+  - Unify hyphen variants (en dash, minus sign → standard hyphen)
+  - Remove or unify punctuation (parentheses, slashes → spaces)
+
+  ## Member Field Mapping
+
+  Maps CSV headers to canonical member fields:
+  - `email` (required)
+  - `first_name` (optional)
+  - `last_name` (optional)
+  - `street` (optional)
+  - `postal_code` (optional)
+  - `city` (optional)
+
+  Supports both English and German variants (e.g., "Email" / "E-Mail", "First Name" / "Vorname").
+
+  ## Custom Field Detection
+
+  Custom fields are detected by matching normalized header names to custom field names.
+  Member fields have priority over custom fields (member field wins in case of collision).
+
+  ## Examples
+
+      iex> HeaderMapper.normalize_header("  E-Mail  ")
+      "e-mail"
+
+      iex> HeaderMapper.build_maps(["Email", "First Name"], [])
+      {:ok, %{member: %{email: 0, first_name: 1}, custom: %{}, unknown: []}}
+
+      iex> HeaderMapper.build_maps(["Email", "CustomField"], [%{id: "cf1", name: "CustomField"}])
+      {:ok, %{member: %{email: 0}, custom: %{"cf1" => 1}, unknown: []}}
+  """
+
+  @type column_map :: %{atom() => non_neg_integer()}
+  @type custom_field_map :: %{String.t() => non_neg_integer()}
+  @type unknown_headers :: [String.t()]
+
+  # Required member fields
+  @required_member_fields [:email]
+
+  # Canonical member fields with their raw variants
+  # These will be normalized at runtime when building the lookup map
+  @member_field_variants_raw %{
+    email: [
+      "email",
+      "e-mail",
+      "e_mail",
+      "e mail",
+      "e-mail adresse",
+      "e-mail-adresse",
+      "mail"
+    ],
+    first_name: [
+      "first name",
+      "firstname",
+      "vorname"
+    ],
+    last_name: [
+      "last name",
+      "lastname",
+      "surname",
+      "nachname",
+      "familienname"
+    ],
+    street: [
+      "street",
+      "address",
+      "strasse"
+    ],
+    postal_code: [
+      "postal code",
+      "postal_code",
+      "zip",
+      "postcode",
+      "plz",
+      "postleitzahl"
+    ],
+    city: [
+      "city",
+      "town",
+      "stadt",
+      "ort"
+    ]
+  }
+
+  # Build reverse map: normalized_variant -> canonical_field
+  # Cached on first access for performance
+  defp normalized_to_canonical do
+    cached = Process.get({__MODULE__, :normalized_to_canonical})
+
+    if cached do
+      cached
+    else
+      map = build_normalized_to_canonical_map()
+      Process.put({__MODULE__, :normalized_to_canonical}, map)
+      map
+    end
+  end
+
+  # Builds the normalized variant -> canonical field map
+  defp build_normalized_to_canonical_map do
+    @member_field_variants_raw
+    |> Enum.flat_map(&map_variants_to_normalized/1)
+    |> Map.new()
+  end
+
+  # Maps a canonical field and its variants to normalized tuples
+  defp map_variants_to_normalized({canonical, variants}) do
+    Enum.map(variants, fn variant ->
+      {normalize_header(variant), canonical}
+    end)
+  end
+
+  @doc """
+  Normalizes a CSV header string for comparison.
+
+  Applies the following transformations:
+  - Trim whitespace
+  - Convert to lowercase
+  - Unicode transliteration (ß → ss, ä → ae, ö → oe, ü → ue)
+  - Unify hyphen variants (en dash U+2013, minus sign U+2212 → standard hyphen)
+  - Remove or unify punctuation (parentheses, slashes → spaces)
+  - Remove all whitespace (ensures "first name" == "firstname")
+  - Final trim
+
+  ## Examples
+
+      iex> normalize_header("  E-Mail  ")
+      "e-mail"
+
+      iex> normalize_header("Straße")
+      "strasse"
+
+      iex> normalize_header("E-Mail (privat)")
+      "e-mailprivat"
+
+      iex> normalize_header("First Name")
+      "firstname"
+
+  """
+  @spec normalize_header(String.t()) :: String.t()
+  def normalize_header(header) when is_binary(header) do
+    header
+    |> String.trim()
+    |> String.downcase()
+    |> transliterate_unicode()
+    |> unify_hyphens()
+    |> normalize_punctuation()
+    |> compress_whitespace()
+    |> String.trim()
+  end
+
+  def normalize_header(_), do: ""
+
+  @doc """
+  Builds column maps for member fields and custom fields from CSV headers.
+
+  ## Parameters
+
+  - `headers` - List of CSV header strings (in column order, 0-based indices)
+  - `custom_fields` - List of custom field maps/structs with at least `:id` and `:name` keys
+
+  ## Returns
+
+  - `{:ok, %{member: column_map, custom: custom_field_map, unknown: unknown_headers}}` on success
+  - `{:error, reason}` on error (missing required field, duplicate headers)
+
+  ## Examples
+
+      iex> build_maps(["Email", "First Name"], [])
+      {:ok, %{member: %{email: 0, first_name: 1}, custom: %{}, unknown: []}}
+
+      iex> build_maps(["Email", "CustomField"], [%{id: "cf1", name: "CustomField"}])
+      {:ok, %{member: %{email: 0}, custom: %{"cf1" => 1}, unknown: []}}
+
+  """
+  @spec build_maps([String.t()], [map()]) ::
+          {:ok, %{member: column_map(), custom: custom_field_map(), unknown: unknown_headers()}}
+          | {:error, String.t()}
+  def build_maps(headers, custom_fields) when is_list(headers) and is_list(custom_fields) do
+    with {:ok, member_map, unknown_after_member} <- build_member_map(headers),
+         {:ok, custom_map, unknown_after_custom} <-
+           build_custom_field_map(headers, unknown_after_member, custom_fields, member_map) do
+      unknown = Enum.map(unknown_after_custom, &Enum.at(headers, &1))
+      {:ok, %{member: member_map, custom: custom_map, unknown: unknown}}
+    end
+  end
+
+  # --- Private Functions ---
+
+  # Transliterates German umlauts and special characters
+  defp transliterate_unicode(str) do
+    str
+    |> String.replace("ß", "ss")
+    |> String.replace("ä", "ae")
+    |> String.replace("ö", "oe")
+    |> String.replace("ü", "ue")
+    |> String.replace("Ä", "ae")
+    |> String.replace("Ö", "oe")
+    |> String.replace("Ü", "ue")
+  end
+
+  # Unifies different hyphen variants to standard hyphen
+  defp unify_hyphens(str) do
+    str
+    # en dash
+    |> String.replace(<<0x2013::utf8>>, "-")
+    # em dash
+    |> String.replace(<<0x2014::utf8>>, "-")
+    # minus sign
+    |> String.replace(<<0x2212::utf8>>, "-")
+  end
+
+  # Normalizes punctuation: parentheses, slashes, underscores become spaces
+  defp normalize_punctuation(str) do
+    str
+    |> String.replace("_", " ")
+    |> String.replace(~r/[()\[\]{}]/, " ")
+    |> String.replace(~r/[\/\\]/, " ")
+  end
+
+  # Compresses multiple whitespace characters to single space, then removes all spaces
+  # This ensures "first name" and "firstname" normalize to the same value
+  defp compress_whitespace(str) do
+    str
+    |> String.replace(~r/\s+/, " ")
+    |> String.replace(" ", "")
+  end
+
+  # Builds member field column map
+  defp build_member_map(headers) do
+    result =
+      headers
+      |> Enum.with_index()
+      |> Enum.reduce_while({%{}, []}, fn {header, index}, {acc_map, acc_unknown} ->
+        normalized = normalize_header(header)
+
+        case process_member_header(header, index, normalized, acc_map, %{}) do
+          {:error, reason} ->
+            {:halt, {:error, reason}}
+
+          {:ok, new_map, _} ->
+            {:cont, {new_map, acc_unknown}}
+
+          {:unknown} ->
+            {:cont, {acc_map, [index | acc_unknown]}}
+        end
+      end)
+
+    case result do
+      {:error, reason} ->
+        {:error, reason}
+
+      {member_map, unknown_indices} ->
+        validate_required_fields(member_map, unknown_indices)
+    end
+  end
+
+  # Processes a single header for member field mapping
+  defp process_member_header(_header, _index, normalized, acc_map, acc_seen)
+       when normalized == "" do
+    {:ok, acc_map, acc_seen}
+  end
+
+  defp process_member_header(_header, index, normalized, acc_map, _acc_seen) do
+    case Map.get(normalized_to_canonical(), normalized) do
+      nil ->
+        {:unknown}
+
+      canonical ->
+        if Map.has_key?(acc_map, canonical) do
+          {:error, "duplicate header for #{canonical} (normalized: #{normalized})"}
+        else
+          {:ok, Map.put(acc_map, canonical, index), %{}}
+        end
+    end
+  end
+
+  # Validates that all required member fields are present
+  defp validate_required_fields(member_map, unknown_indices) do
+    missing_required =
+      @required_member_fields
+      |> Enum.filter(&(not Map.has_key?(member_map, &1)))
+
+    if Enum.empty?(missing_required) do
+      {:ok, member_map, Enum.reverse(unknown_indices)}
+    else
+      missing_field = List.first(missing_required)
+      variants = Map.get(@member_field_variants_raw, missing_field, [])
+      accepted = Enum.join(variants, ", ")
+
+      {:error, "Missing required header: #{missing_field} (accepted: #{accepted})"}
+    end
+  end
+
+  # Builds custom field column map from unmatched headers
+  defp build_custom_field_map(headers, unknown_indices, custom_fields, _member_map) do
+    custom_field_lookup = build_custom_field_lookup(custom_fields)
+
+    result =
+      unknown_indices
+      |> Enum.reduce_while({%{}, []}, fn index, {acc_map, acc_unknown} ->
+        header = Enum.at(headers, index)
+        normalized = normalize_header(header)
+
+        case process_custom_field_header(
+               header,
+               index,
+               normalized,
+               custom_field_lookup,
+               acc_map,
+               %{}
+             ) do
+          {:error, reason} ->
+            {:halt, {:error, reason}}
+
+          {:ok, new_map, _} ->
+            {:cont, {new_map, acc_unknown}}
+
+          {:unknown} ->
+            {:cont, {acc_map, [index | acc_unknown]}}
+        end
+      end)
+
+    case result do
+      {:error, reason} ->
+        {:error, reason}
+
+      {custom_map, remaining_unknown} ->
+        {:ok, custom_map, Enum.reverse(remaining_unknown)}
+    end
+  end
+
+  # Builds normalized custom field name -> id lookup map
+  defp build_custom_field_lookup(custom_fields) do
+    custom_fields
+    |> Enum.reduce(%{}, fn cf, acc ->
+      name = Map.get(cf, :name) || Map.get(cf, "name")
+      id = Map.get(cf, :id) || Map.get(cf, "id")
+
+      if name && id do
+        normalized_name = normalize_header(name)
+        Map.put(acc, normalized_name, id)
+      else
+        acc
+      end
+    end)
+  end
+
+  # Processes a single header for custom field mapping
+  defp process_custom_field_header(
+         _header,
+         _index,
+         normalized,
+         _custom_field_lookup,
+         acc_map,
+         _acc_seen
+       )
+       when normalized == "" do
+    {:ok, acc_map, %{}}
+  end
+
+  defp process_custom_field_header(
+         _header,
+         index,
+         normalized,
+         custom_field_lookup,
+         acc_map,
+         _acc_seen
+       ) do
+    if Map.has_key?(custom_field_lookup, normalized) do
+      custom_field_id = custom_field_lookup[normalized]
+
+      if Map.has_key?(acc_map, custom_field_id) do
+        {:error, "duplicate custom field header (normalized: #{normalized})"}
+      else
+        {:ok, Map.put(acc_map, custom_field_id, index), %{}}
+      end
+    else
+      {:unknown}
+    end
+  end
+end
diff --git a/lib/mv/membership/import/member_csv.ex b/lib/mv/membership/import/member_csv.ex
index 9e30a20..26756b4 100644
--- a/lib/mv/membership/import/member_csv.ex
+++ b/lib/mv/membership/import/member_csv.ex
@@ -2,6 +2,8 @@ defmodule Mv.Membership.Import.MemberCSV do
   @moduledoc """
   Service module for importing members from CSV files.
 
+  require Ash.Query
+
   This module provides the core API for CSV member import functionality:
   - `prepare/2` - Parses and validates CSV content, returns import state
   - `process_chunk/3` - Processes a chunk of rows and creates members
@@ -61,6 +63,7 @@ defmodule Mv.Membership.Import.MemberCSV do
           chunks: list(list({pos_integer(), map()})),
           column_map: %{atom() => non_neg_integer()},
           custom_field_map: %{String.t() => non_neg_integer()},
+          custom_field_lookup: %{String.t() => %{id: String.t(), value_type: atom()}},
           warnings: list(String.t())
         }
 
@@ -70,6 +73,9 @@ defmodule Mv.Membership.Import.MemberCSV do
           errors: list(Error.t())
         }
 
+  alias Mv.Membership.Import.CsvParser
+  alias Mv.Membership.Import.HeaderMapper
+
   @doc """
   Prepares CSV content for import by parsing, mapping headers, and validating limits.
 
@@ -104,12 +110,133 @@ defmodule Mv.Membership.Import.MemberCSV do
   """
   @spec prepare(String.t(), keyword()) :: {:ok, import_state()} | {:error, String.t()}
   def prepare(file_content, opts \\ []) do
-    # TODO: Implement in Issue #3 (CSV Parsing)
-    # This is a skeleton implementation that will be filled in later
-    _ = {file_content, opts}
+    max_rows = Keyword.get(opts, :max_rows, 1000)
+    chunk_size = Keyword.get(opts, :chunk_size, 200)
 
-    # Placeholder return - will be replaced with actual implementation
-    {:error, "Not yet implemented"}
+    with {:ok, headers, rows} <- CsvParser.parse(file_content),
+         {:ok, custom_fields} <- load_custom_fields(),
+         {:ok, maps, warnings} <- build_header_maps(headers, custom_fields),
+         :ok <- validate_row_count(rows, max_rows) do
+      chunks = chunk_rows(rows, maps, chunk_size)
+
+      # Build custom field lookup for efficient value processing
+      custom_field_lookup = build_custom_field_lookup(custom_fields)
+
+      {:ok,
+       %{
+         chunks: chunks,
+         column_map: maps.member,
+         custom_field_map: maps.custom,
+         custom_field_lookup: custom_field_lookup,
+         warnings: warnings
+       }}
+    end
+  end
+
+  # Loads all custom fields from the database
+  defp load_custom_fields do
+    custom_fields =
+      Mv.Membership.CustomField
+      |> Ash.read!()
+
+    {:ok, custom_fields}
+  rescue
+    e ->
+      {:error, "Failed to load custom fields: #{Exception.message(e)}"}
+  end
+
+  # Builds custom field lookup map for efficient value processing
+  defp build_custom_field_lookup(custom_fields) do
+    custom_fields
+    |> Enum.reduce(%{}, fn cf, acc ->
+      id_str = to_string(cf.id)
+      Map.put(acc, id_str, %{id: cf.id, value_type: cf.value_type})
+    end)
+  end
+
+  # Builds header maps using HeaderMapper and collects warnings for unknown custom fields
+  defp build_header_maps(headers, custom_fields) do
+    # Convert custom fields to maps with id and name
+    custom_field_maps =
+      Enum.map(custom_fields, fn cf ->
+        %{id: to_string(cf.id), name: cf.name}
+      end)
+
+    case HeaderMapper.build_maps(headers, custom_field_maps) do
+      {:ok, %{member: member_map, custom: custom_map, unknown: unknown}} ->
+        # Build warnings for unknown custom field columns
+        warnings =
+          unknown
+          |> Enum.filter(fn header ->
+            # Check if it could be a custom field (not a known member field)
+            normalized = HeaderMapper.normalize_header(header)
+            # If it's not empty and not a member field, it might be a custom field
+            normalized != "" && not member_field?(normalized)
+          end)
+          |> Enum.map(fn header ->
+            "Unknown column '#{header}' will be ignored. " <>
+              "If this is a custom field, create it in Mila before importing."
+          end)
+
+        {:ok, %{member: member_map, custom: custom_map}, warnings}
+
+      {:error, reason} ->
+        {:error, reason}
+    end
+  end
+
+  # Checks if a normalized header matches a member field
+  # Uses HeaderMapper's internal logic to check if header would map to a member field
+  defp member_field?(normalized) do
+    # Try to build maps with just this header - if it maps to a member field, it's a member field
+    case HeaderMapper.build_maps([normalized], []) do
+      {:ok, %{member: member_map}} ->
+        # If member_map is not empty, it's a member field
+        map_size(member_map) > 0
+
+      _ ->
+        false
+    end
+  end
+
+  # Validates that row count doesn't exceed limit
+  defp validate_row_count(rows, max_rows) do
+    if length(rows) > max_rows do
+      {:error, "CSV file exceeds maximum row limit of #{max_rows} rows"}
+    else
+      :ok
+    end
+  end
+
+  # Chunks rows and converts them to row maps using column maps
+  defp chunk_rows(rows, maps, chunk_size) do
+    rows
+    |> Enum.chunk_every(chunk_size)
+    |> Enum.map(fn chunk ->
+      Enum.map(chunk, fn {line_number, row_values} ->
+        row_map = build_row_map(row_values, maps)
+        {line_number, row_map}
+      end)
+    end)
+  end
+
+  # Builds a row map from raw row values using column maps
+  defp build_row_map(row_values, maps) do
+    member_map =
+      maps.member
+      |> Enum.reduce(%{}, fn {field, index}, acc ->
+        value = Enum.at(row_values, index, "")
+        Map.put(acc, field, value)
+      end)
+
+    custom_map =
+      maps.custom
+      |> Enum.reduce(%{}, fn {custom_field_id, index}, acc ->
+        value = Enum.at(row_values, index, "")
+        Map.put(acc, custom_field_id, value)
+      end)
+
+    %{member: member_map, custom: custom_map}
   end
 
   @doc """
@@ -126,8 +253,9 @@ defmodule Mv.Membership.Import.MemberCSV do
 
   - `chunk_rows_with_lines` - List of tuples `{csv_line_number, row_map}` where:
     - `csv_line_number` - Physical line number in CSV (1-based)
-    - `row_map` - Map of column names to values
-  - `column_map` - Map of canonical field names (atoms) to column indices
+    - `row_map` - Map with `:member` and `:custom` keys containing field values
+  - `column_map` - Map of canonical field names (atoms) to column indices (for reference)
+  - `custom_field_map` - Map of custom field IDs (strings) to column indices (for reference)
   - `opts` - Optional keyword list for processing options
 
   ## Returns
@@ -137,22 +265,212 @@ defmodule Mv.Membership.Import.MemberCSV do
 
   ## Examples
 
-      iex> chunk = [{2, %{"email" => "john@example.com"}}]
+      iex> chunk = [{2, %{member: %{email: "john@example.com"}, custom: %{}}}]
       iex> column_map = %{email: 0}
-      iex> MemberCSV.process_chunk(chunk, column_map)
+      iex> custom_field_map = %{}
+      iex> MemberCSV.process_chunk(chunk, column_map, custom_field_map)
       {:ok, %{inserted: 1, failed: 0, errors: []}}
   """
   @spec process_chunk(
           list({pos_integer(), map()}),
           %{atom() => non_neg_integer()},
+          %{String.t() => non_neg_integer()},
           keyword()
         ) :: {:ok, chunk_result()} | {:error, String.t()}
-  def process_chunk(chunk_rows_with_lines, column_map, opts \\ []) do
-    # TODO: Implement in Issue #6 (Persistence)
-    # This is a skeleton implementation that will be filled in later
-    _ = {chunk_rows_with_lines, column_map, opts}
+  def process_chunk(chunk_rows_with_lines, _column_map, _custom_field_map, opts \\ []) do
+    custom_field_lookup = Keyword.get(opts, :custom_field_lookup, %{})
 
-    # Placeholder return - will be replaced with actual implementation
-    {:ok, %{inserted: 0, failed: 0, errors: []}}
+    {inserted, failed, errors} =
+      Enum.reduce(chunk_rows_with_lines, {0, 0, []}, fn {line_number, row_map},
+                                                        {acc_inserted, acc_failed, acc_errors} ->
+        case process_row(row_map, line_number, custom_field_lookup) do
+          {:ok, _member} ->
+            {acc_inserted + 1, acc_failed, acc_errors}
+
+          {:error, error} ->
+            {acc_inserted, acc_failed + 1, [error | acc_errors]}
+        end
+      end)
+
+    {:ok, %{inserted: inserted, failed: failed, errors: Enum.reverse(errors)}}
   end
+
+  # Processes a single row and creates member with custom field values
+  defp process_row(
+         %{member: member_attrs, custom: custom_attrs},
+         line_number,
+         custom_field_lookup
+       ) do
+    # Prepare custom field values for Ash
+    custom_field_values = prepare_custom_field_values(custom_attrs, custom_field_lookup)
+
+    # Create member with custom field values
+    member_attrs_with_cf =
+      member_attrs
+      |> Map.put(:custom_field_values, custom_field_values)
+      |> trim_string_values()
+
+    # Only include custom_field_values if not empty
+    final_attrs =
+      if Enum.empty?(custom_field_values) do
+        Map.delete(member_attrs_with_cf, :custom_field_values)
+      else
+        member_attrs_with_cf
+      end
+
+    case Mv.Membership.create_member(final_attrs) do
+      {:ok, member} ->
+        {:ok, member}
+
+      {:error, %Ash.Error.Invalid{} = error} ->
+        {:error, format_ash_error(error, line_number)}
+
+      {:error, error} ->
+        {:error, %Error{csv_line_number: line_number, field: nil, message: inspect(error)}}
+    end
+  rescue
+    e ->
+      {:error, %Error{csv_line_number: line_number, field: nil, message: Exception.message(e)}}
+  end
+
+  # Prepares custom field values from row map for Ash
+  defp prepare_custom_field_values(custom_attrs, custom_field_lookup) when is_map(custom_attrs) do
+    custom_attrs
+    |> Enum.filter(fn {_id, value} -> value != nil && value != "" end)
+    |> Enum.map(fn {custom_field_id_str, value} ->
+      case Map.get(custom_field_lookup, custom_field_id_str) do
+        nil ->
+          # Custom field not found, skip
+          nil
+
+        %{id: custom_field_id, value_type: value_type} ->
+          %{
+            "custom_field_id" => to_string(custom_field_id),
+            "value" => format_custom_field_value(value, value_type)
+          }
+      end
+    end)
+    |> Enum.filter(&(&1 != nil))
+  end
+
+  defp prepare_custom_field_values(_, _), do: []
+
+  # Formats a custom field value according to its type
+  # Uses _union_type and _union_value format as expected by Ash
+  defp format_custom_field_value(value, :string) when is_binary(value) do
+    %{"_union_type" => "string", "_union_value" => String.trim(value)}
+  end
+
+  defp format_custom_field_value(value, :integer) when is_binary(value) do
+    case Integer.parse(value) do
+      {int_value, _} -> %{"_union_type" => "integer", "_union_value" => int_value}
+      :error -> %{"_union_type" => "string", "_union_value" => String.trim(value)}
+    end
+  end
+
+  defp format_custom_field_value(value, :boolean) when is_binary(value) do
+    bool_value =
+      value
+      |> String.trim()
+      |> String.downcase()
+      |> case do
+        "true" -> true
+        "1" -> true
+        "yes" -> true
+        "ja" -> true
+        _ -> false
+      end
+
+    %{"_union_type" => "boolean", "_union_value" => bool_value}
+  end
+
+  defp format_custom_field_value(value, :date) when is_binary(value) do
+    case Date.from_iso8601(String.trim(value)) do
+      {:ok, date} -> %{"_union_type" => "date", "_union_value" => date}
+      {:error, _} -> %{"_union_type" => "string", "_union_value" => String.trim(value)}
+    end
+  end
+
+  defp format_custom_field_value(value, :email) when is_binary(value) do
+    %{"_union_type" => "email", "_union_value" => String.trim(value)}
+  end
+
+  defp format_custom_field_value(value, _type) when is_binary(value) do
+    # Default to string if type is unknown
+    %{"_union_type" => "string", "_union_value" => String.trim(value)}
+  end
+
+  # Trims all string values in member attributes
+  defp trim_string_values(attrs) do
+    Enum.reduce(attrs, %{}, fn {key, value}, acc ->
+      trimmed_value =
+        if is_binary(value) do
+          String.trim(value)
+        else
+          value
+        end
+
+      Map.put(acc, key, trimmed_value)
+    end)
+  end
+
+  # Formats Ash errors into MemberCSV.Error structs
+  defp format_ash_error(%Ash.Error.Invalid{errors: errors}, line_number) do
+    # Try to find email-related errors first (for better error messages)
+    email_error =
+      Enum.find(errors, fn error ->
+        case error do
+          %{field: :email} -> true
+          _ -> false
+        end
+      end)
+
+    case email_error || List.first(errors) do
+      %{field: field, message: message} when is_atom(field) ->
+        %Error{
+          csv_line_number: line_number,
+          field: field,
+          message: format_error_message(message, field)
+        }
+
+      %{message: message} ->
+        %Error{
+          csv_line_number: line_number,
+          field: nil,
+          message: format_error_message(message, nil)
+        }
+
+      _ ->
+        %Error{
+          csv_line_number: line_number,
+          field: nil,
+          message: "Validation failed"
+        }
+    end
+  end
+
+  # Formats error messages, handling common cases like email uniqueness
+  defp format_error_message(message, field) when is_binary(message) do
+    if email_uniqueness_error?(message, field) do
+      "email has already been taken"
+    else
+      message
+    end
+  end
+
+  defp format_error_message(message, _field), do: to_string(message)
+
+  # Checks if error message indicates email uniqueness constraint violation
+  defp email_uniqueness_error?(message, :email) do
+    message_lower = String.downcase(message)
+
+    String.contains?(message_lower, "unique") or
+      String.contains?(message_lower, "constraint") or
+      String.contains?(message_lower, "duplicate") or
+      String.contains?(message_lower, "already been taken") or
+      String.contains?(message_lower, "already exists") or
+      String.contains?(message_lower, "violates unique constraint")
+  end
+
+  defp email_uniqueness_error?(_message, _field), do: false
 end
diff --git a/test/mv/membership/import/header_mapper_test.exs b/test/mv/membership/import/header_mapper_test.exs
new file mode 100644
index 0000000..5e7efbd
--- /dev/null
+++ b/test/mv/membership/import/header_mapper_test.exs
@@ -0,0 +1,244 @@
+defmodule Mv.Membership.Import.HeaderMapperTest do
+  use ExUnit.Case, async: true
+
+  alias Mv.Membership.Import.HeaderMapper
+
+  describe "normalize_header/1" do
+    test "trims whitespace" do
+      assert HeaderMapper.normalize_header("  email  ") == "email"
+    end
+
+    test "converts to lowercase" do
+      assert HeaderMapper.normalize_header("EMAIL") == "email"
+      assert HeaderMapper.normalize_header("E-Mail") == "e-mail"
+    end
+
+    test "normalizes Unicode characters" do
+      # ß -> ss
+      assert HeaderMapper.normalize_header("Straße") == "strasse"
+      # Umlaute transliteration (ä -> ae, ö -> oe, ü -> ue)
+      assert HeaderMapper.normalize_header("Müller") == "mueller"
+      assert HeaderMapper.normalize_header("Köln") == "koeln"
+      assert HeaderMapper.normalize_header("Grün") == "gruen"
+    end
+
+    test "compresses and removes whitespace" do
+      # Whitespace is removed entirely to ensure "first name" == "firstname"
+      assert HeaderMapper.normalize_header("first  name") == "firstname"
+      assert HeaderMapper.normalize_header("email   address") == "emailaddress"
+    end
+
+    test "unifies hyphen variants" do
+      # Different Unicode hyphen characters should become standard hyphen
+      # en dash
+      assert HeaderMapper.normalize_header("E–Mail") == "e-mail"
+      # minus sign
+      assert HeaderMapper.normalize_header("E−Mail") == "e-mail"
+      # standard hyphen
+      assert HeaderMapper.normalize_header("E-Mail") == "e-mail"
+    end
+
+    test "removes or unifies punctuation" do
+      # Parentheses, slashes, etc. are removed (whitespace is also removed)
+      assert HeaderMapper.normalize_header("E-Mail (privat)") == "e-mailprivat"
+      assert HeaderMapper.normalize_header("Telefon / Mobil") == "telefonmobil"
+    end
+
+    test "handles empty strings" do
+      assert HeaderMapper.normalize_header("") == ""
+      assert HeaderMapper.normalize_header("   ") == ""
+    end
+  end
+
+  describe "build_maps/2" do
+    test "maps English email variant correctly" do
+      headers = ["Email"]
+
+      assert {:ok, %{member: member_map, custom: custom_map, unknown: unknown}} =
+               HeaderMapper.build_maps(headers, [])
+
+      assert member_map[:email] == 0
+      assert custom_map == %{}
+      assert unknown == []
+    end
+
+    test "maps German email variant correctly" do
+      headers = ["E-Mail"]
+
+      assert {:ok, %{member: member_map, custom: custom_map, unknown: unknown}} =
+               HeaderMapper.build_maps(headers, [])
+
+      assert member_map[:email] == 0
+      assert custom_map == %{}
+      assert unknown == []
+    end
+
+    test "maps multiple member fields" do
+      headers = ["Email", "First Name", "Last Name"]
+
+      assert {:ok, %{member: member_map, custom: custom_map, unknown: unknown}} =
+               HeaderMapper.build_maps(headers, [])
+
+      assert member_map[:email] == 0
+      assert member_map[:first_name] == 1
+      assert member_map[:last_name] == 2
+      assert custom_map == %{}
+      assert unknown == []
+    end
+
+    test "handles Unicode and whitespace in headers" do
+      headers = ["  E-Mail  ", "Straße", "  Telefon / Mobil  "]
+
+      assert {:ok, %{member: member_map, custom: custom_map, unknown: unknown}} =
+               HeaderMapper.build_maps(headers, [])
+
+      assert member_map[:email] == 0
+      assert member_map[:street] == 1
+      # "Telefon / Mobil" is not a known member field, so it should be unknown
+      assert length(unknown) == 1
+      assert custom_map == %{}
+    end
+
+    test "returns error when duplicate headers normalize to same field" do
+      headers = ["Email", "E-Mail"]
+
+      assert {:error, reason} = HeaderMapper.build_maps(headers, [])
+      assert reason =~ "duplicate"
+      assert reason =~ "email"
+    end
+
+    test "returns error when required field email is missing" do
+      headers = ["First Name", "Last Name"]
+
+      assert {:error, reason} = HeaderMapper.build_maps(headers, [])
+      assert reason =~ "Missing required header"
+      assert reason =~ "email"
+      assert reason =~ "accepted"
+    end
+
+    test "collects unknown columns" do
+      headers = ["Email", "FooBar", "UnknownColumn"]
+
+      assert {:ok, %{member: member_map, custom: custom_map, unknown: unknown}} =
+               HeaderMapper.build_maps(headers, [])
+
+      assert member_map[:email] == 0
+      assert length(unknown) == 2
+      assert "FooBar" in unknown or "foobar" in unknown
+      assert "UnknownColumn" in unknown or "unknowncolumn" in unknown
+      assert custom_map == %{}
+    end
+
+    test "ignores empty headers after normalization" do
+      headers = ["Email", "   ", ""]
+
+      assert {:ok, %{member: member_map, custom: custom_map, unknown: unknown}} =
+               HeaderMapper.build_maps(headers, [])
+
+      assert member_map[:email] == 0
+      assert custom_map == %{}
+      assert unknown == []
+    end
+
+    test "maps custom field columns correctly" do
+      headers = ["Email", "Lieblingsfarbe"]
+      custom_fields = [%{id: "cf1", name: "Lieblingsfarbe"}]
+
+      assert {:ok, %{member: member_map, custom: custom_map, unknown: unknown}} =
+               HeaderMapper.build_maps(headers, custom_fields)
+
+      assert member_map[:email] == 0
+      assert custom_map["cf1"] == 1
+      assert unknown == []
+    end
+
+    test "custom field collision: member field wins" do
+      headers = ["Email"]
+      # Custom field with name "Email" should not override member field
+      custom_fields = [%{id: "cf1", name: "Email"}]
+
+      assert {:ok, %{member: member_map, custom: custom_map, unknown: unknown}} =
+               HeaderMapper.build_maps(headers, custom_fields)
+
+      assert member_map[:email] == 0
+      # Custom field should not be in custom_map because member field has priority
+      assert custom_map == %{}
+      assert unknown == []
+    end
+
+    test "handles custom field with Unicode normalization" do
+      headers = ["Email", "Straße"]
+      custom_fields = [%{id: "cf1", name: "Straße"}]
+
+      assert {:ok, %{member: member_map, custom: custom_map, unknown: unknown}} =
+               HeaderMapper.build_maps(headers, custom_fields)
+
+      assert member_map[:email] == 0
+      # "Straße" is a member field (street), so it should be in member_map, not custom_map
+      assert member_map[:street] == 1
+      assert custom_map == %{}
+      assert unknown == []
+    end
+
+    test "handles unknown custom field columns" do
+      headers = ["Email", "UnknownCustomField"]
+      custom_fields = [%{id: "cf1", name: "KnownField"}]
+
+      assert {:ok, %{member: member_map, custom: custom_map, unknown: unknown}} =
+               HeaderMapper.build_maps(headers, custom_fields)
+
+      assert member_map[:email] == 0
+      assert custom_map == %{}
+      # UnknownCustomField should be in unknown list
+      assert length(unknown) == 1
+    end
+
+    test "handles duplicate custom field names after normalization" do
+      headers = ["Email", "CustomField", "Custom Field"]
+      custom_fields = [%{id: "cf1", name: "CustomField"}]
+
+      # Both "CustomField" and "Custom Field" normalize to the same, so this should error
+      assert {:error, reason} = HeaderMapper.build_maps(headers, custom_fields)
+      assert reason =~ "duplicate"
+    end
+
+    test "maps all supported member fields" do
+      headers = [
+        "Email",
+        "First Name",
+        "Last Name",
+        "Street",
+        "Postal Code",
+        "City"
+      ]
+
+      assert {:ok, %{member: member_map, custom: custom_map, unknown: unknown}} =
+               HeaderMapper.build_maps(headers, [])
+
+      assert member_map[:email] == 0
+      assert member_map[:first_name] == 1
+      assert member_map[:last_name] == 2
+      assert member_map[:street] == 3
+      assert member_map[:postal_code] == 4
+      assert member_map[:city] == 5
+      assert custom_map == %{}
+      assert unknown == []
+    end
+
+    test "maps German member field variants" do
+      headers = ["E-Mail", "Vorname", "Nachname", "Straße", "PLZ", "Stadt"]
+
+      assert {:ok, %{member: member_map, custom: custom_map, unknown: unknown}} =
+               HeaderMapper.build_maps(headers, [])
+
+      assert member_map[:email] == 0
+      assert member_map[:first_name] == 1
+      assert member_map[:last_name] == 2
+      assert member_map[:street] == 3
+      assert member_map[:postal_code] == 4
+      assert member_map[:city] == 5
+      assert custom_map == %{}
+      assert unknown == []
+    end
+  end
+end
diff --git a/test/mv/membership/import/member_csv_test.exs b/test/mv/membership/import/member_csv_test.exs
index e918afd..356cc19 100644
--- a/test/mv/membership/import/member_csv_test.exs
+++ b/test/mv/membership/import/member_csv_test.exs
@@ -44,7 +44,6 @@ defmodule Mv.Membership.Import.MemberCSVTest do
       assert match?({:ok, _}, result) or match?({:error, _}, result)
     end
 
-    @tag :skip
     test "returns {:ok, import_state} on success" do
       file_content = "email\njohn@example.com"
       opts = []
@@ -56,6 +55,8 @@ defmodule Mv.Membership.Import.MemberCSVTest do
       assert Map.has_key?(import_state, :column_map)
       assert Map.has_key?(import_state, :custom_field_map)
       assert Map.has_key?(import_state, :warnings)
+      assert import_state.column_map[:email] == 0
+      assert import_state.chunks != []
     end
 
     test "returns {:error, reason} on failure" do
@@ -71,24 +72,183 @@ defmodule Mv.Membership.Import.MemberCSVTest do
     end
   end
 
-  describe "process_chunk/3" do
-    test "function exists and accepts chunk_rows_with_lines, column_map, and opts" do
-      chunk_rows_with_lines = [{2, %{"email" => "john@example.com"}}]
+  describe "process_chunk/4" do
+    test "function exists and accepts chunk_rows_with_lines, column_map, custom_field_map, and opts" do
+      chunk_rows_with_lines = [{2, %{member: %{email: "john@example.com"}, custom: %{}}}]
       column_map = %{email: 0}
+      custom_field_map = %{}
       opts = []
 
       # This will fail until the function is implemented
-      result = MemberCSV.process_chunk(chunk_rows_with_lines, column_map, opts)
+      result = MemberCSV.process_chunk(chunk_rows_with_lines, column_map, custom_field_map, opts)
       assert match?({:ok, _}, result) or match?({:error, _}, result)
     end
 
-    test "returns {:ok, chunk_result} on success" do
-      chunk_rows_with_lines = [{2, %{"email" => "john@example.com"}}]
-      column_map = %{email: 0}
+    test "creates member successfully with valid data" do
+      chunk_rows_with_lines = [
+        {2, %{member: %{email: "john@example.com", first_name: "John"}, custom: %{}}}
+      ]
+
+      column_map = %{email: 0, first_name: 1}
+      custom_field_map = %{}
       opts = []
 
       assert {:ok, chunk_result} =
-               MemberCSV.process_chunk(chunk_rows_with_lines, column_map, opts)
+               MemberCSV.process_chunk(chunk_rows_with_lines, column_map, custom_field_map, opts)
+
+      assert chunk_result.inserted == 1
+      assert chunk_result.failed == 0
+      assert chunk_result.errors == []
+
+      # Verify member was created
+      members = Mv.Membership.list_members!()
+      assert Enum.any?(members, &(&1.email == "john@example.com"))
+    end
+
+    test "returns error for invalid email" do
+      chunk_rows_with_lines = [
+        {2, %{member: %{email: "invalid-email"}, custom: %{}}}
+      ]
+
+      column_map = %{email: 0}
+      custom_field_map = %{}
+      opts = []
+
+      assert {:ok, chunk_result} =
+               MemberCSV.process_chunk(chunk_rows_with_lines, column_map, custom_field_map, opts)
+
+      assert chunk_result.inserted == 0
+      assert chunk_result.failed == 1
+      assert length(chunk_result.errors) == 1
+
+      error = List.first(chunk_result.errors)
+      assert error.csv_line_number == 2
+      assert error.field == :email
+      assert error.message =~ "email"
+    end
+
+    test "returns error for duplicate email" do
+      # Create existing member first
+      {:ok, _existing} =
+        Mv.Membership.create_member(%{email: "duplicate@example.com", first_name: "Existing"})
+
+      chunk_rows_with_lines = [
+        {2, %{member: %{email: "duplicate@example.com", first_name: "New"}, custom: %{}}}
+      ]
+
+      column_map = %{email: 0, first_name: 1}
+      custom_field_map = %{}
+      opts = []
+
+      assert {:ok, chunk_result} =
+               MemberCSV.process_chunk(chunk_rows_with_lines, column_map, custom_field_map, opts)
+
+      assert chunk_result.inserted == 0
+      assert chunk_result.failed == 1
+      assert length(chunk_result.errors) == 1
+
+      error = List.first(chunk_result.errors)
+      assert error.csv_line_number == 2
+      assert error.field == :email
+      assert error.message =~ "email" or error.message =~ "duplicate" or error.message =~ "unique"
+    end
+
+    test "creates member with custom field values" do
+      # Create custom field first
+      {:ok, custom_field} =
+        Mv.Membership.CustomField
+        |> Ash.Changeset.for_create(:create, %{
+          name: "Phone",
+          value_type: :string
+        })
+        |> Ash.create()
+
+      chunk_rows_with_lines = [
+        {2,
+         %{
+           member: %{email: "withcustom@example.com"},
+           custom: %{to_string(custom_field.id) => "123-456-7890"}
+         }}
+      ]
+
+      column_map = %{email: 0}
+      custom_field_map = %{to_string(custom_field.id) => 1}
+
+      custom_field_lookup = %{
+        to_string(custom_field.id) => %{id: custom_field.id, value_type: custom_field.value_type}
+      }
+
+      opts = [custom_field_lookup: custom_field_lookup]
+
+      assert {:ok, chunk_result} =
+               MemberCSV.process_chunk(chunk_rows_with_lines, column_map, custom_field_map, opts)
+
+      assert chunk_result.inserted == 1
+      assert chunk_result.failed == 0
+
+      # Verify member and custom field value were created
+      members = Mv.Membership.list_members!()
+      member = Enum.find(members, &(&1.email == "withcustom@example.com"))
+      assert member != nil
+
+      {:ok, member_with_cf} = Ash.load(member, :custom_field_values)
+      assert length(member_with_cf.custom_field_values) == 1
+      cfv = List.first(member_with_cf.custom_field_values)
+      assert cfv.custom_field_id == custom_field.id
+      assert cfv.value.value == "123-456-7890"
+    end
+
+    test "handles multiple rows with mixed success and failure" do
+      chunk_rows_with_lines = [
+        {2, %{member: %{email: "valid1@example.com"}, custom: %{}}},
+        {3, %{member: %{email: "invalid-email"}, custom: %{}}},
+        {4, %{member: %{email: "valid2@example.com"}, custom: %{}}}
+      ]
+
+      column_map = %{email: 0}
+      custom_field_map = %{}
+      opts = []
+
+      assert {:ok, chunk_result} =
+               MemberCSV.process_chunk(chunk_rows_with_lines, column_map, custom_field_map, opts)
+
+      assert chunk_result.inserted == 2
+      assert chunk_result.failed == 1
+      assert length(chunk_result.errors) == 1
+
+      error = List.first(chunk_result.errors)
+      assert error.csv_line_number == 3
+    end
+
+    test "preserves CSV line numbers in errors" do
+      chunk_rows_with_lines = [
+        {5, %{member: %{email: "invalid"}, custom: %{}}},
+        {10, %{member: %{email: "also-invalid"}, custom: %{}}}
+      ]
+
+      column_map = %{email: 0}
+      custom_field_map = %{}
+      opts = []
+
+      assert {:ok, chunk_result} =
+               MemberCSV.process_chunk(chunk_rows_with_lines, column_map, custom_field_map, opts)
+
+      assert chunk_result.failed == 2
+      assert length(chunk_result.errors) == 2
+
+      line_numbers = Enum.map(chunk_result.errors, & &1.csv_line_number)
+      assert 5 in line_numbers
+      assert 10 in line_numbers
+    end
+
+    test "returns {:ok, chunk_result} on success" do
+      chunk_rows_with_lines = [{2, %{member: %{email: "test@example.com"}, custom: %{}}}]
+      column_map = %{email: 0}
+      custom_field_map = %{}
+      opts = []
+
+      assert {:ok, chunk_result} =
+               MemberCSV.process_chunk(chunk_rows_with_lines, column_map, custom_field_map, opts)
 
       # Check that chunk_result contains expected fields
       assert Map.has_key?(chunk_result, :inserted)
@@ -99,19 +259,23 @@ defmodule Mv.Membership.Import.MemberCSVTest do
       assert is_list(chunk_result.errors)
     end
 
-    test "returns {:error, reason} on failure" do
+    test "returns {:ok, _} with zero counts for empty chunk" do
       chunk_rows_with_lines = []
       column_map = %{}
+      custom_field_map = %{}
       opts = []
 
-      # This might return {:ok, _} with zero counts or {:error, _}
-      result = MemberCSV.process_chunk(chunk_rows_with_lines, column_map, opts)
-      assert match?({:ok, _}, result) or match?({:error, _}, result)
+      assert {:ok, chunk_result} =
+               MemberCSV.process_chunk(chunk_rows_with_lines, column_map, custom_field_map, opts)
+
+      assert chunk_result.inserted == 0
+      assert chunk_result.failed == 0
+      assert chunk_result.errors == []
     end
 
     test "function has documentation" do
       # Check that @doc exists by reading the module
-      assert function_exported?(MemberCSV, :process_chunk, 3)
+      assert function_exported?(MemberCSV, :process_chunk, 4)
     end
   end