diff --git a/src/wormhole/observer.py b/src/wormhole/observer.py
new file mode 100644
index 0000000..99a22a0
--- /dev/null
+++ b/src/wormhole/observer.py
@@ -0,0 +1,69 @@
+from __future__ import unicode_literals, print_function
+from twisted.internet.defer import Deferred
+from twisted.python.failure import Failure
+
+NoResult = object()
+
+class OneShotObserver(object):
+    def __init__(self, eventual_queue):
+        self._eq = eventual_queue
+        self._result = NoResult
+        self._observers = [] # list of Deferreds
+
+    def when_fired(self):
+        d = Deferred()
+        self._observers.append(d)
+        self._maybe_call_observers()
+        return d
+
+    def fire(self, result):
+        assert self._result is NoResult
+        self._result = result
+        self._maybe_call_observers()
+
+    def _maybe_call_observers(self):
+        if self._result is NoResult:
+            return
+        observers, self._observers = self._observers, []
+        for d in observers:
+            self._eq.eventually(d.callback, self._result)
+
+    def error(self, f):
+        # errors will override an existing result
+        assert isinstance(f, Failure)
+        self._result = f
+        self._maybe_call_observers()
+
+    def fire_if_not_fired(self, result):
+        if self._result is NoResult:
+            self.fire(result)
+
+class SequenceObserver(object):
+    def __init__(self, eventual_queue):
+        self._eq = eventual_queue
+        self._error = None
+        self._results = []
+        self._observers = []
+
+    def when_next_event(self):
+        d = Deferred()
+        if self._error:
+            self._eq.eventually(d.errback, self._error)
+        elif self._results:
+            result = self._results.pop(0)
+            self._eq.eventually(d.callback, result)
+        else:
+            self._observers.append(d)
+        return d
+
+    def fire(self, result):
+        if isinstance(result, Failure):
+            self._error = result
+            for d in self._observers:
+                self._eq.eventually(d.errback, self._error)
+            self._observers = []
+        else:
+            self._results.append(result)
+            if self._observers:
+                d = self._observers.pop(0)
+                self._eq.eventually(d.callback, self._results.pop(0))
diff --git a/src/wormhole/test/test_observer.py b/src/wormhole/test/test_observer.py
new file mode 100644
index 0000000..d974cc5
--- /dev/null
+++ b/src/wormhole/test/test_observer.py
@@ -0,0 +1,122 @@
+from twisted.trial import unittest
+from twisted.internet.task import Clock
+from twisted.python.failure import Failure
+from ..eventual import EventualQueue
+from ..observer import OneShotObserver, SequenceObserver
+
+class OneShot(unittest.TestCase):
+    def test_fire(self):
+        c = Clock()
+        eq = EventualQueue(c)
+        o = OneShotObserver(eq)
+        res = object()
+        d1 = o.when_fired()
+        eq.flush_sync()
+        self.assertNoResult(d1)
+        o.fire(res)
+        eq.flush_sync()
+        self.assertIdentical(self.successResultOf(d1), res)
+        d2 = o.when_fired()
+        eq.flush_sync()
+        self.assertIdentical(self.successResultOf(d2), res)
+        o.fire_if_not_fired(object())
+        eq.flush_sync()
+
+    def test_fire_if_not_fired(self):
+        c = Clock()
+        eq = EventualQueue(c)
+        o = OneShotObserver(eq)
+        res1 = object()
+        res2 = object()
+        d1 = o.when_fired()
+        eq.flush_sync()
+        self.assertNoResult(d1)
+        o.fire_if_not_fired(res1)
+        o.fire_if_not_fired(res2)
+        eq.flush_sync()
+        self.assertIdentical(self.successResultOf(d1), res1)
+
+    def test_error_before_firing(self):
+        c = Clock()
+        eq = EventualQueue(c)
+        o = OneShotObserver(eq)
+        f = Failure(ValueError("oops"))
+        d1 = o.when_fired()
+        eq.flush_sync()
+        self.assertNoResult(d1)
+        o.error(f)
+        eq.flush_sync()
+        self.assertIdentical(self.failureResultOf(d1), f)
+        d2 = o.when_fired()
+        eq.flush_sync()
+        self.assertIdentical(self.failureResultOf(d2), f)
+
+    def test_error_after_firing(self):
+        c = Clock()
+        eq = EventualQueue(c)
+        o = OneShotObserver(eq)
+        res = object()
+        f = Failure(ValueError("oops"))
+
+        o.fire(res)
+        eq.flush_sync()
+        d1 = o.when_fired()
+        o.error(f)
+        d2 = o.when_fired()
+        eq.flush_sync()
+        self.assertIdentical(self.successResultOf(d1), res)
+        self.assertIdentical(self.failureResultOf(d2), f)
+
+
+class Sequence(unittest.TestCase):
+    def test_fire(self):
+        c = Clock()
+        eq = EventualQueue(c)
+        o = SequenceObserver(eq)
+        d1 = o.when_next_event()
+        eq.flush_sync()
+        self.assertNoResult(d1)
+        d2 = o.when_next_event()
+        eq.flush_sync()
+        self.assertNoResult(d1)
+        self.assertNoResult(d2)
+
+        ev1 = object()
+        ev2 = object()
+        o.fire(ev1)
+        eq.flush_sync()
+        self.assertIdentical(self.successResultOf(d1), ev1)
+        self.assertNoResult(d2)
+
+        o.fire(ev2)
+        eq.flush_sync()
+        self.assertIdentical(self.successResultOf(d2), ev2)
+
+        ev3 = object()
+        ev4 = object()
+        o.fire(ev3)
+        o.fire(ev4)
+
+        d3 = o.when_next_event()
+        eq.flush_sync()
+        self.assertIdentical(self.successResultOf(d3), ev3)
+
+        d4 = o.when_next_event()
+        eq.flush_sync()
+        self.assertIdentical(self.successResultOf(d4), ev4)
+
+    def test_error(self):
+        c = Clock()
+        eq = EventualQueue(c)
+        o = SequenceObserver(eq)
+        d1 = o.when_next_event()
+        eq.flush_sync()
+        self.assertNoResult(d1)
+        f = Failure(ValueError("oops"))
+        o.fire(f)
+        eq.flush_sync()
+        self.assertIdentical(self.failureResultOf(d1), f)
+        d2 = o.when_next_event()
+        eq.flush_sync()
+        self.assertIdentical(self.failureResultOf(d2), f)
+