diff --git a/lib/channel/waitgroup.rb b/lib/channel/waitgroup.rb
new file mode 100644
index 0000000..38f2c50
--- /dev/null
+++ b/lib/channel/waitgroup.rb
@@ -0,0 +1,38 @@
+class WaitGroup
+  def initialize
+    @mutex = Mutex.new
+    @count = 0
+    @waiting = []
+  end
+
+  def add(count)
+    sync! { @count += count }
+  end
+
+  def done
+    sync! do
+      fail 'negative count' if done?
+      @count -= 1
+      wake!
+    end
+  end
+
+  private def done?
+    @count == 0
+  end
+
+  def wait
+    sync! do
+      @waiting << Thread.current
+      @mutex.sleep until done?
+    end
+  end
+
+  private def wake!
+    @waiting.each { |t| t.wakeup if t.alive? }
+  end
+
+  private def sync!(&block)
+    @mutex.synchronize(&block)
+  end
+end
diff --git a/test/test_waitgroup.rb b/test/test_waitgroup.rb
new file mode 100644
index 0000000..867436c
--- /dev/null
+++ b/test/test_waitgroup.rb
@@ -0,0 +1,50 @@
+require 'test/unit'
+require 'thread'
+require 'channel/waitgroup'
+
+class TestWaitGroup < Test::Unit::TestCase
+  module Util
+    def meanwhile(*procs, &blk)
+      threads = procs.map { |p| Thread.new(&p) }
+      blk.call
+      threads.each(&:join)
+    end
+  end
+
+  module Assert
+    def assert_raise_with_message(exc, msg, &block)
+      e = assert_raise(exc, &block)
+      assert_match(msg, e.message)
+    end
+  end
+
+  include Util
+  include Assert
+
+  def test_waitgroup
+    Time.now.tap do |start|
+      wg = WaitGroup.new
+      wg.add(5)
+      ok1 = false
+      ok2 = false
+      ok3 = false
+      ok4 = false
+      ok5 = false
+      go -> { sleep 0.1; ok1 = true; wg.done }
+      go -> { sleep 0.2; ok2 = true; wg.done }
+      go -> { sleep 0.3; ok3 = true; wg.done }
+      go -> { sleep 0.4; ok4 = true; wg.done }
+      go -> { sleep 0.5; ok5 = true; wg.done }
+      wg.wait
+      duration = Time.now - start
+      assert_equal(true, duration > 0.45)
+      assert_equal(true, duration < 0.70)
+      assert_true(ok1)
+      assert_true(ok2)
+      assert_true(ok3)
+      assert_true(ok4)
+      assert_true(ok5)
+      assert_raises(RuntimeError) { wg.done }
+    end
+  end
+end