diff --git a/lib/x/client.rb b/lib/x/client.rb
index 7a2c917..453112c 100644
--- a/lib/x/client.rb
+++ b/lib/x/client.rb
@@ -8,6 +8,7 @@
 require_relative "redirect_handler"
 require_relative "request_builder"
 require_relative "response_parser"
+require_relative "users"
 
 module X
   # A client for interacting with the X API
@@ -15,6 +16,7 @@ module X
   class Client
     extend Forwardable
     include ClientCredentials
+    include Users
 
     # Default base URL for the X API
     DEFAULT_BASE_URL = "https://api.twitter.com/2/".freeze
diff --git a/lib/x/users.rb b/lib/x/users.rb
new file mode 100644
index 0000000..6da0cf8
--- /dev/null
+++ b/lib/x/users.rb
@@ -0,0 +1,110 @@
+module X
+  # Methods for the Users Lookup API
+  # @see https://docs.x.com/x-api/users/lookup/introduction
+  # @api public
+  module Users
+    # Retrieve a single user by ID
+    #
+    # @param id [String, Integer] the user ID
+    # @param user_fields [Array<String>, String, nil] list of user fields to return
+    # @param expansions [Array<String>, String, nil] list of expansions to include
+    # @param tweet_fields [Array<String>, String, nil] list of tweet fields to return (requires pinned_tweet_id expansion)
+    # @return [Hash] the parsed response body
+    # @see https://docs.x.com/x-api/users/lookup/api-reference/get-users-id
+    # @example Retrieve a user with default fields
+    #   client.user("123456")
+    # @example Retrieve a user with additional fields
+    #   client.user("123456", user_fields: ["created_at", "description", "public_metrics"])
+    def user(id, user_fields: nil, expansions: nil, tweet_fields: nil)
+      get(build_users_endpoint("users/#{id}", user_fields:, expansions:, tweet_fields:))
+    end
+
+    # Retrieve multiple users by their IDs
+    #
+    # @param ids [Array<String>, Array<Integer>] list of user IDs (up to 100)
+    # @param user_fields [Array<String>, String, nil] list of user fields to return
+    # @param expansions [Array<String>, String, nil] list of expansions to include
+    # @param tweet_fields [Array<String>, String, nil] list of tweet fields to return (requires pinned_tweet_id expansion)
+    # @return [Hash] the parsed response body
+    # @see https://docs.x.com/x-api/users/lookup/api-reference/get-users
+    # @example Retrieve multiple users
+    #   client.users(ids: ["123456", "789012"])
+    # @example Retrieve multiple users with fields
+    #   client.users(ids: ["123456", "789012"], user_fields: ["created_at", "public_metrics"])
+    def users(ids:, user_fields: nil, expansions: nil, tweet_fields: nil)
+      params = assemble_users_params(user_fields:, expansions:, tweet_fields:)
+      params["ids"] = Array(ids).join(",")
+      get("users?#{URI.encode_www_form(params)}")
+    end
+
+    # Retrieve a single user by username
+    #
+    # @param username [String] the username (without @ prefix)
+    # @param user_fields [Array<String>, String, nil] list of user fields to return
+    # @param expansions [Array<String>, String, nil] list of expansions to include
+    # @param tweet_fields [Array<String>, String, nil] list of tweet fields to return (requires pinned_tweet_id expansion)
+    # @return [Hash] the parsed response body
+    # @see https://docs.x.com/x-api/users/lookup/api-reference/get-users-by-username-username
+    # @example Retrieve a user by username
+    #   client.user_by_username("sferik")
+    # @example Retrieve a user by username with fields
+    #   client.user_by_username("sferik", user_fields: ["created_at", "description"])
+    def user_by_username(username, user_fields: nil, expansions: nil, tweet_fields: nil)
+      get(build_users_endpoint("users/by/username/#{username}", user_fields:, expansions:, tweet_fields:))
+    end
+
+    # Retrieve multiple users by their usernames
+    #
+    # @param usernames [Array<String>] list of usernames (up to 100, without @ prefix)
+    # @param user_fields [Array<String>, String, nil] list of user fields to return
+    # @param expansions [Array<String>, String, nil] list of expansions to include
+    # @param tweet_fields [Array<String>, String, nil] list of tweet fields to return (requires pinned_tweet_id expansion)
+    # @return [Hash] the parsed response body
+    # @see https://docs.x.com/x-api/users/lookup/api-reference/get-users-by
+    # @example Retrieve users by usernames
+    #   client.users_by_usernames(usernames: ["sferik", "xdevelopers"])
+    # @example Retrieve users by usernames with fields
+    #   client.users_by_usernames(usernames: ["sferik", "xdevelopers"], user_fields: ["created_at"])
+    def users_by_usernames(usernames:, user_fields: nil, expansions: nil, tweet_fields: nil)
+      params = assemble_users_params(user_fields:, expansions:, tweet_fields:)
+      params["usernames"] = Array(usernames).join(",")
+      get("users/by?#{URI.encode_www_form(params)}")
+    end
+
+    # Retrieve the authenticated user
+    #
+    # @param user_fields [Array<String>, String, nil] list of user fields to return
+    # @param expansions [Array<String>, String, nil] list of expansions to include
+    # @param tweet_fields [Array<String>, String, nil] list of tweet fields to return (requires pinned_tweet_id expansion)
+    # @return [Hash] the parsed response body
+    # @see https://docs.x.com/x-api/users/lookup/api-reference/get-users-me
+    # @example Retrieve the authenticated user
+    #   client.me
+    # @example Retrieve the authenticated user with fields
+    #   client.me(user_fields: ["created_at", "description", "public_metrics"])
+    def me(user_fields: nil, expansions: nil, tweet_fields: nil)
+      get(build_users_endpoint("users/me", user_fields:, expansions:, tweet_fields:))
+    end
+
+    private
+
+    # Build an endpoint path with optional query parameters
+    # @api private
+    def build_users_endpoint(path, user_fields: nil, expansions: nil, tweet_fields: nil)
+      params = assemble_users_params(user_fields:, expansions:, tweet_fields:)
+      return path if params.empty?
+
+      "#{path}?#{URI.encode_www_form(params)}"
+    end
+
+    # Assemble query parameters hash from field options
+    # @api private
+    def assemble_users_params(user_fields: nil, expansions: nil, tweet_fields: nil)
+      params = {}
+      params["user.fields"] = Array(user_fields).join(",") if user_fields
+      params["expansions"] = Array(expansions).join(",") if expansions
+      params["tweet.fields"] = Array(tweet_fields).join(",") if tweet_fields
+      params
+    end
+  end
+end
diff --git a/test/x/users_test.rb b/test/x/users_test.rb
new file mode 100644
index 0000000..bdd2cec
--- /dev/null
+++ b/test/x/users_test.rb
@@ -0,0 +1,171 @@
+require_relative "../test_helper"
+
+module X
+  class UsersTest < Minitest::Test
+    cover Users
+
+    BASE_URL = "https://api.twitter.com/2/"
+
+    def setup
+      @client = Client.new(bearer_token: TEST_BEARER_TOKEN)
+      @user_data = {"data" => {"id" => "123", "name" => "Test", "username" => "test"}}.freeze
+      @users_data = {"data" => [{"id" => "123", "name" => "Test", "username" => "test"}]}.freeze
+      @headers = {"Content-Type" => "application/json"}
+    end
+
+    def test_user
+      stub_request(:get, "#{BASE_URL}users/123")
+        .to_return(body: @user_data.to_json, headers: @headers)
+
+      result = @client.user("123")
+
+      assert_equal @user_data, result
+    end
+
+    def test_user_with_user_fields
+      stub_request(:get, "#{BASE_URL}users/123?user.fields=created_at,description")
+        .to_return(body: @user_data.to_json, headers: @headers)
+
+      result = @client.user("123", user_fields: %w[created_at description])
+
+      assert_equal @user_data, result
+    end
+
+    def test_user_with_expansions
+      stub_request(:get, "#{BASE_URL}users/123?expansions=pinned_tweet_id")
+        .to_return(body: @user_data.to_json, headers: @headers)
+
+      result = @client.user("123", expansions: ["pinned_tweet_id"])
+
+      assert_equal @user_data, result
+    end
+
+    def test_user_with_tweet_fields
+      stub_request(:get, "#{BASE_URL}users/123?tweet.fields=created_at,text")
+        .to_return(body: @user_data.to_json, headers: @headers)
+
+      result = @client.user("123", tweet_fields: %w[created_at text])
+
+      assert_equal @user_data, result
+    end
+
+    def test_user_with_all_fields
+      stub_request(:get, "#{BASE_URL}users/123?user.fields=created_at&expansions=pinned_tweet_id&tweet.fields=text")
+        .to_return(body: @user_data.to_json, headers: @headers)
+
+      result = @client.user("123", user_fields: ["created_at"], expansions: ["pinned_tweet_id"],
+        tweet_fields: ["text"])
+
+      assert_equal @user_data, result
+    end
+
+    def test_user_with_string_fields
+      stub_request(:get, "#{BASE_URL}users/123?user.fields=created_at")
+        .to_return(body: @user_data.to_json, headers: @headers)
+
+      result = @client.user("123", user_fields: "created_at")
+
+      assert_equal @user_data, result
+    end
+
+    def test_users
+      stub_request(:get, "#{BASE_URL}users?ids=123,456")
+        .to_return(body: @users_data.to_json, headers: @headers)
+
+      result = @client.users(ids: %w[123 456])
+
+      assert_equal @users_data, result
+    end
+
+    def test_users_with_user_fields
+      stub_request(:get, "#{BASE_URL}users?user.fields=created_at,description&ids=123,456")
+        .to_return(body: @users_data.to_json, headers: @headers)
+
+      result = @client.users(ids: %w[123 456], user_fields: %w[created_at description])
+
+      assert_equal @users_data, result
+    end
+
+    def test_users_with_single_id
+      stub_request(:get, "#{BASE_URL}users?ids=123")
+        .to_return(body: @users_data.to_json, headers: @headers)
+
+      result = @client.users(ids: "123")
+
+      assert_equal @users_data, result
+    end
+
+    def test_user_by_username
+      stub_request(:get, "#{BASE_URL}users/by/username/sferik")
+        .to_return(body: @user_data.to_json, headers: @headers)
+
+      result = @client.user_by_username("sferik")
+
+      assert_equal @user_data, result
+    end
+
+    def test_user_by_username_with_user_fields
+      stub_request(:get, "#{BASE_URL}users/by/username/sferik?user.fields=created_at,public_metrics")
+        .to_return(body: @user_data.to_json, headers: @headers)
+
+      result = @client.user_by_username("sferik", user_fields: %w[created_at public_metrics])
+
+      assert_equal @user_data, result
+    end
+
+    def test_users_by_usernames
+      stub_request(:get, "#{BASE_URL}users/by?usernames=sferik,xdevelopers")
+        .to_return(body: @users_data.to_json, headers: @headers)
+
+      result = @client.users_by_usernames(usernames: %w[sferik xdevelopers])
+
+      assert_equal @users_data, result
+    end
+
+    def test_users_by_usernames_with_user_fields
+      stub_request(:get, "#{BASE_URL}users/by?user.fields=created_at&usernames=sferik,xdevelopers")
+        .to_return(body: @users_data.to_json, headers: @headers)
+
+      result = @client.users_by_usernames(usernames: %w[sferik xdevelopers], user_fields: ["created_at"])
+
+      assert_equal @users_data, result
+    end
+
+    def test_users_by_usernames_with_single_username
+      stub_request(:get, "#{BASE_URL}users/by?usernames=sferik")
+        .to_return(body: @users_data.to_json, headers: @headers)
+
+      result = @client.users_by_usernames(usernames: "sferik")
+
+      assert_equal @users_data, result
+    end
+
+    def test_me
+      stub_request(:get, "#{BASE_URL}users/me")
+        .to_return(body: @user_data.to_json, headers: @headers)
+
+      result = @client.me
+
+      assert_equal @user_data, result
+    end
+
+    def test_me_with_user_fields
+      stub_request(:get, "#{BASE_URL}users/me?user.fields=created_at,description,public_metrics")
+        .to_return(body: @user_data.to_json, headers: @headers)
+
+      result = @client.me(user_fields: %w[created_at description public_metrics])
+
+      assert_equal @user_data, result
+    end
+
+    def test_me_with_all_fields
+      stub_request(:get, "#{BASE_URL}users/me?user.fields=created_at&expansions=pinned_tweet_id&tweet.fields=text")
+        .to_return(body: @user_data.to_json, headers: @headers)
+
+      result = @client.me(user_fields: ["created_at"], expansions: ["pinned_tweet_id"],
+        tweet_fields: ["text"])
+
+      assert_equal @user_data, result
+    end
+  end
+end