diff --git a/lib/gc_index_relay/nostr/filter.ex b/lib/gc_index_relay/nostr/filter.ex
index 577bb0d..432a1b1 100644
--- a/lib/gc_index_relay/nostr/filter.ex
+++ b/lib/gc_index_relay/nostr/filter.ex
@@ -1,11 +1,42 @@
 defmodule GcIndexRelay.Nostr.Filter do
+  alias GcIndexRelay.Nostr.Event
   alias GcIndexRelay.Repo
+  alias GcIndexRelay.Nostr.Tag
   import Ecto.Query
 
-  @derive [Jason.Encoder]
   defstruct [:ids, :authors, :kinds, :tags, :since, :until, :limit]
 
-  # TODO: Implement tag filtering
+  @type t :: %__MODULE__{
+          ids: [String.t()] | nil,
+          authors: [String.t()] | nil,
+          kinds: [integer()] | nil,
+          tags: %{String.t() => [String.t()]} | nil,
+          since: integer() | nil,
+          until: integer() | nil,
+          limit: pos_integer() | nil
+        }
+
+  @spec from_map(map()) :: t()
+  def from_map(map) when is_map(map) do
+    tags =
+      for {"#" <> k, v} <- map,
+          do: {k, v},
+          into: %{}
+
+    %__MODULE__{
+      ids: map["ids"] || nil,
+      authors: map["authors"] || nil,
+      kinds: map["kinds"] || nil,
+      tags:
+        case tags do
+          tags when map_size(tags) == 0 -> nil
+          tags -> tags
+        end,
+      since: map["since"] || nil,
+      until: map["until"] || nil,
+      limit: map["limit"] || nil
+    }
+  end
 
   @doc """
   Applies a filter to an Ecto query of Nostr events.
@@ -19,7 +50,8 @@ defmodule GcIndexRelay.Nostr.Filter do
 
   A filtered list of events in descending order of creation time.
   """
-  def apply(%Ecto.Query{} = query, %__MODULE__{} = filter) do
+  @spec apply(Ecto.Query.t(), t()) :: [struct()]
+  def apply(%Ecto.Query{from: %{source: {_table, Event}}} = query, %__MODULE__{} = filter) do
     query
     |> apply_ids(filter.ids)
     |> apply_authors(filter.authors)
@@ -27,30 +59,81 @@ defmodule GcIndexRelay.Nostr.Filter do
     |> apply_since(filter.since)
     |> apply_until(filter.until)
     |> preload(:tags)
+    |> apply_tags(filter.tags)
     # Always sort in descending order of creation time
     |> order_by([e], desc: e.created_at)
     |> apply_limit(filter.limit)
     |> Repo.all()
   end
 
+  @spec apply_ids(Ecto.Query.t(), [String.t()] | nil) :: Ecto.Query.t()
   defp apply_ids(query, nil), do: query
   defp apply_ids(query, []), do: query
   defp apply_ids(query, ids), do: where(query, [e], e.id in ^ids)
 
+  @spec apply_authors(Ecto.Query.t(), [String.t()] | nil) :: Ecto.Query.t()
   defp apply_authors(query, nil), do: query
   defp apply_authors(query, []), do: query
   defp apply_authors(query, authors), do: where(query, [e], e.author in ^authors)
 
+  @spec apply_kinds(Ecto.Query.t(), [integer()] | nil) :: Ecto.Query.t()
   defp apply_kinds(query, nil), do: query
   defp apply_kinds(query, []), do: query
   defp apply_kinds(query, kinds), do: where(query, [e], e.kind in ^kinds)
 
+  @spec apply_since(Ecto.Query.t(), integer() | nil) :: Ecto.Query.t()
   defp apply_since(query, nil), do: query
   defp apply_since(query, since), do: where(query, [e], e.created_at >= ^since)
 
+  @spec apply_until(Ecto.Query.t(), integer() | nil) :: Ecto.Query.t()
   defp apply_until(query, nil), do: query
   defp apply_until(query, until), do: where(query, [e], e.created_at <= ^until)
 
+  @spec apply_tags(Ecto.Query.t(), map() | nil) :: Ecto.Query.t()
+  defp apply_tags(query, nil), do: query
+  defp apply_tags(query, tags) when map_size(tags) == 0, do: query
+
+  defp apply_tags(query, tags) do
+    query = from(e in query, as: :event_query)
+
+    Enum.reduce(tags, query, fn {tag_name, tag_values}, acc_query ->
+      where(
+        acc_query,
+        [e],
+        exists(
+          from t in Tag,
+            where: t.event_id == parent_as(:event_query).id,
+            where: t.name == ^tag_name,
+            where: t.value in ^tag_values
+        )
+      )
+    end)
+  end
+
+  @spec apply_limit(Ecto.Query.t(), pos_integer() | nil) :: Ecto.Query.t()
   defp apply_limit(query, nil), do: query
   defp apply_limit(query, limit), do: limit(query, ^limit)
 end
+
+defimpl Jason.Encoder, for: GcIndexRelay.Nostr.Filter do
+  alias GcIndexRelay.Nostr.Filter
+
+  def encode(%Filter{} = filter, opts) do
+    # Prefix single-letter tags with '#'
+    tags_map =
+      case filter.tags do
+        nil -> %{}
+        tags -> Enum.map(tags, fn {k, v} -> {"#" <> k, v} end) |> Map.new()
+      end
+
+    # Produce a map from the remaining filter fields
+    rest_map =
+      filter
+      |> Map.from_struct()
+      |> Map.delete(:tags)
+
+    # Merge the tags into the remaining fields and encode with Jason
+    Map.merge(rest_map, tags_map)
+    |> Jason.Encode.map(opts)
+  end
+end