diff --git a/Steepfile b/Steepfile
index ff6c47b..ad37cb0 100644
--- a/Steepfile
+++ b/Steepfile
@@ -6,6 +6,7 @@ rbs_root = Gem.loaded_specs['rbs'].full_gem_path
 stdlib_path core_root: 'core', stdlib_root: "#{rbs_root}/stdlib"
 target :test do
   check 'test'
+  signature 'test/sig'
 
   configure_code_diagnostics(D::Ruby.strict)
 end
diff --git a/core/enumerable.rbs b/core/enumerable.rbs
new file mode 100644
index 0000000..3cdf8d9
--- /dev/null
+++ b/core/enumerable.rbs
@@ -0,0 +1,12 @@
+module Enumerable[unchecked out Elem] : _Each[Elem]
+  interface _Each[unchecked out T]
+    def each: () { (T) -> void } -> void
+  end
+
+  # TODO: Array
+  # def collect: [T]() { (Elem item) -> T } -> Array[T]
+  #
+  # alias map collect
+
+  def each_with_index: () { (Elem item, Integer index) -> void } -> self
+end
diff --git a/test/enumerable.rb b/test/enumerable.rb
new file mode 100644
index 0000000..e1039c0
--- /dev/null
+++ b/test/enumerable.rb
@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+# rubocop:disable Lint/Void
+# rubocop:disable Style/Documentation
+
+class EnumerableTestClass
+  include Enumerable
+
+  def each
+    yield 'Hello'
+  end
+end
+
+a = EnumerableTestClass.new
+a.each { |item| item }
+
+# TODO: Array
+# a.collect { |item| item }
+#
+# a.map { |item| item }
+
+a.each_with_index do |item, index|
+  item
+  index
+end
+
+# rubocop:enable all
diff --git a/test/sig/enumerable.rbs b/test/sig/enumerable.rbs
new file mode 100644
index 0000000..85e27aa
--- /dev/null
+++ b/test/sig/enumerable.rbs
@@ -0,0 +1,5 @@
+class EnumerableTestClass
+  include Enumerable[String]
+
+  def each: () { (String) -> void } -> void
+end