diff --git a/.coveragerc b/.coveragerc
index bc0afe5c..29bc9dec 100644
--- a/.coveragerc
+++ b/.coveragerc
@@ -1,4 +1,5 @@
 [run]
+branch = True
 source = zope.interface
 
 [report]
@@ -8,4 +9,3 @@ exclude_lines =
     class I[A-Z]\w+\((Interface|I[A-Z].*)\):
     raise NotImplementedError
     self\.fail
-    @_skip_under_py2
diff --git a/.gitignore b/.gitignore
index f261c9d5..35946f41 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,6 +3,7 @@
 *.so
 __pycache__
 .coverage
+.coverage.*
 .installed.cfg
 nosetests.xml
 coverage.xml
diff --git a/src/zope/interface/_compat.py b/src/zope/interface/_compat.py
index b9c19dcb..fb61e13b 100644
--- a/src/zope/interface/_compat.py
+++ b/src/zope/interface/_compat.py
@@ -16,7 +16,7 @@
 import sys
 import types
 
-if sys.version_info[0] < 3: #pragma NO COVER
+if sys.version_info[0] < 3:
 
     def _normalize_name(name):
         if isinstance(name, basestring):
@@ -31,7 +31,7 @@ def _normalize_name(name):
     PYTHON3 = False
     PYTHON2 = True
 
-else: # pragma: no cover
+else:
 
     def _normalize_name(name):
         if isinstance(name, bytes):
@@ -48,11 +48,11 @@ def _normalize_name(name):
     PYTHON3 = True
     PYTHON2 = False
 
-def _skip_under_py3k(test_method): # pragma: no cover
+def _skip_under_py3k(test_method):
     import unittest
     return unittest.skipIf(sys.version_info[0] >= 3, "Only on Python 2")(test_method)
 
 
-def _skip_under_py2(test_method): # pragma: no cover
+def _skip_under_py2(test_method):
     import unittest
     return unittest.skipIf(sys.version_info[0] < 3, "Only on Python 3")(test_method)
diff --git a/src/zope/interface/adapter.py b/src/zope/interface/adapter.py
index b39ceba3..d4825b2a 100644
--- a/src/zope/interface/adapter.py
+++ b/src/zope/interface/adapter.py
@@ -410,7 +410,7 @@ def subscriptions(self, required, provided):
 
 try:
     from zope.interface._zope_interface_coptimizations import LookupBase
-except ImportError: # pragma: no cover
+except ImportError:
     LookupBase = LookupBaseFallback
 
 
@@ -446,7 +446,7 @@ def subscriptions(self, required, provided):
 
 try:
     from zope.interface._zope_interface_coptimizations import VerifyingBase
-except ImportError: # pragma: no cover
+except ImportError:
     VerifyingBase = VerifyingBaseFallback
 
 
diff --git a/src/zope/interface/advice.py b/src/zope/interface/advice.py
index 6bf4815a..e55930d5 100644
--- a/src/zope/interface/advice.py
+++ b/src/zope/interface/advice.py
@@ -28,9 +28,9 @@
 from types import FunctionType
 try:
     from types import ClassType
-except ImportError:  # pragma: no cover Python > 3.x
+except ImportError:
     __python3 = True
-else: # pragma: no cover Python < 3.x
+else:
     __python3 = False
 
 import sys
diff --git a/src/zope/interface/declarations.py b/src/zope/interface/declarations.py
index 13433bd8..acea1207 100644
--- a/src/zope/interface/declarations.py
+++ b/src/zope/interface/declarations.py
@@ -436,8 +436,6 @@ def __call__(self, ob):
 def _implements(name, interfaces, classImplements):
     # This entire approach is invalid under Py3K.  Don't even try to fix
     # the coverage for this block there. :(
-    if PYTHON3: # pragma: no cover
-        raise TypeError('Class advice impossible in Python3')
     frame = sys._getframe(2)
     locals = frame.f_locals
 
@@ -481,7 +479,7 @@ def implements(*interfaces):
     """
     # This entire approach is invalid under Py3K.  Don't even try to fix
     # the coverage for this block there. :(
-    if PYTHON3: # pragma: no cover
+    if PYTHON3:
         raise TypeError(_ADVICE_ERROR % 'implementer')
     _implements("implements", interfaces, classImplements)
 
@@ -509,7 +507,7 @@ def implementsOnly(*interfaces):
     """
     # This entire approach is invalid under Py3K.  Don't even try to fix
     # the coverage for this block there. :(
-    if PYTHON3: # pragma: no cover
+    if PYTHON3:
         raise TypeError(_ADVICE_ERROR % 'implementer_only')
     _implements("implementsOnly", interfaces, classImplementsOnly)
 
@@ -581,7 +579,7 @@ def directlyProvides(object, *interfaces):
         # Note that we can't get here from Py3k tests:  there is no normal
         # class which isn't descriptor aware.
         if not isinstance(object,
-                          DescriptorAwareMetaClasses): # pragma: no cover Py3k
+                          DescriptorAwareMetaClasses):
             raise TypeError("Attempt to make an interface declaration on a "
                             "non-descriptor-aware class")
 
@@ -641,9 +639,9 @@ def __get__(self, inst, cls):
 # Try to get C base:
 try:
     import zope.interface._zope_interface_coptimizations
-except ImportError:  # pragma: no cover
+except ImportError:
     pass
-else:  # pragma: no cover
+else:
     from zope.interface._zope_interface_coptimizations import ClassProvidesBase
 
 
@@ -715,7 +713,7 @@ def classProvides(*interfaces):
     # This entire approach is invalid under Py3K.  Don't even try to fix
     # the coverage for this block there. :(
 
-    if PYTHON3: # pragma: no cover
+    if PYTHON3:
         raise TypeError(_ADVICE_ERROR % 'provider')
 
     frame = sys._getframe(1)
@@ -918,9 +916,9 @@ def _normalizeargs(sequence, output = None):
 
 try:
     import zope.interface._zope_interface_coptimizations
-except ImportError: # pragma: no cover
+except ImportError:
     pass
-else: # pragma: no cover PyPy
+else:
     from zope.interface._zope_interface_coptimizations import implementedBy
     from zope.interface._zope_interface_coptimizations import providedBy
     from zope.interface._zope_interface_coptimizations import (
diff --git a/src/zope/interface/interface.py b/src/zope/interface/interface.py
index 382f14cb..e44f93f4 100644
--- a/src/zope/interface/interface.py
+++ b/src/zope/interface/interface.py
@@ -115,7 +115,7 @@ def isOrExtends(self, interface):
 SpecificationBase = SpecificationBasePy
 try:
     from zope.interface._zope_interface_coptimizations import SpecificationBase
-except ImportError: # pragma: no cover
+except ImportError:
     pass
 
 _marker = object()
@@ -156,14 +156,14 @@ def __adapt__(self, obj):
 InterfaceBase = InterfaceBasePy
 try:
     from zope.interface._zope_interface_coptimizations import InterfaceBase
-except ImportError: # pragma: no cover
+except ImportError:
     pass
 
 
 adapter_hooks = []
 try:
     from zope.interface._zope_interface_coptimizations import adapter_hooks
-except ImportError: # pragma: no cover
+except ImportError:
     pass
 
 
diff --git a/src/zope/interface/tests/advisory_testing.py b/src/zope/interface/tests/advisory_testing.py
index 30dadc55..b159e937 100644
--- a/src/zope/interface/tests/advisory_testing.py
+++ b/src/zope/interface/tests/advisory_testing.py
@@ -32,7 +32,7 @@ def pong(klass):
     class ClassicClass:
         __metaclass__ = ClassType
         classLevelFrameInfo = getFrameInfo(sys._getframe())
-except ImportError: # pragma: no cover (Py3, this module may not even be imported)
+except ImportError:
     ClassicClass = None
 
 class NewStyleClass:
diff --git a/src/zope/interface/tests/odd.py b/src/zope/interface/tests/odd.py
index 7789a76c..74c61584 100644
--- a/src/zope/interface/tests/odd.py
+++ b/src/zope/interface/tests/odd.py
@@ -72,7 +72,7 @@ def __getattribute__(cls, name):
         if name == '__class__':
             return cls
         # Under Python 3.6, __prepare__ gets requested
-        return type.__getattribute__(cls, name) # pragma: no cover
+        return type.__getattribute__(cls, name)
 
 
 class MetaClass(object):
diff --git a/src/zope/interface/tests/test_adapter.py b/src/zope/interface/tests/test_adapter.py
index 24c33346..7420e1c6 100644
--- a/src/zope/interface/tests/test_adapter.py
+++ b/src/zope/interface/tests/test_adapter.py
@@ -558,7 +558,7 @@ def test_optimizations(self):
         from zope.interface.adapter import LookupBaseFallback
         try:
             import zope.interface._zope_interface_coptimizations
-        except ImportError: # pragma: no cover (pypy)
+        except ImportError:
             self.assertIs(self._getTargetClass(), LookupBaseFallback)
         else:
             self.assertIsNot(self._getTargetClass(), LookupBaseFallback)
@@ -729,7 +729,7 @@ def test_optimizations(self):
         from zope.interface.adapter import VerifyingBaseFallback
         try:
             import zope.interface._zope_interface_coptimizations
-        except ImportError: # pragma: no cover (pypy)
+        except ImportError:
             self.assertIs(self._getTargetClass(), VerifyingBaseFallback)
         else:
             self.assertIsNot(self._getTargetClass(), VerifyingBaseFallback)
@@ -1390,7 +1390,7 @@ def test__normalize_name_str(self):
         STR = b'str'
         if sys.version_info[0] < 3:
             self.assertEqual(_normalize_name(STR), unicode(STR))
-        else: # pragma: no cover (tox runs coverage on Python 2)
+        else:
             self.assertEqual(_normalize_name(STR), str(STR, 'ascii'))
 
     def test__normalize_name_unicode(self):
diff --git a/src/zope/interface/tests/test_declarations.py b/src/zope/interface/tests/test_declarations.py
index 05b09a53..43f95c82 100644
--- a/src/zope/interface/tests/test_declarations.py
+++ b/src/zope/interface/tests/test_declarations.py
@@ -31,7 +31,7 @@ def _run_generated_code(self, code, globs, locs,
                 exec(code, globs, locs)
                 self.assertEqual(len(log), 0) # no longer warn
                 return True
-            else: # pragma: no cover (tox runs coverage on Python 2)
+            else:
                 try:
                     exec(code, globs, locs)
                 except TypeError:
@@ -339,10 +339,14 @@ class Foo(object):
 
     def test_dictless_wo_existing_Implements_cant_assign___implemented__(self):
         class Foo(object):
-            def _get_impl(self): return None
-            def _set_impl(self, val): raise TypeError
+            def _get_impl(self):
+                raise NotImplementedError()
+            def _set_impl(self, val):
+                raise TypeError
             __implemented__ = property(_get_impl, _set_impl)
-            def __call__(self): pass  #act like a factory
+            def __call__(self):
+                # act like a factory
+                raise NotImplementedError()
         foo = Foo()
         self.assertRaises(TypeError, self._callFUT, foo)
 
@@ -479,7 +483,7 @@ def test_optimizations(self):
         from zope.interface.declarations import implementedBy
         try:
             import zope.interface._zope_interface_coptimizations
-        except ImportError: # pragma: no cover (pypy)
+        except ImportError:
             self.assertIs(implementedBy, implementedByFallback)
         else:
             self.assertIsNot(implementedBy, implementedByFallback)
@@ -685,7 +689,8 @@ def test_function(self):
         from zope.interface.interface import InterfaceClass
         IFoo = InterfaceClass('IFoo')
         decorator = self._makeOne(IFoo)
-        def _function(): pass
+        def _function():
+            raise NotImplementedError()
         self.assertRaises(ValueError, decorator, _function)
 
     def test_method(self):
@@ -693,7 +698,8 @@ def test_method(self):
         IFoo = InterfaceClass('IFoo')
         decorator = self._makeOne(IFoo)
         class Bar:
-            def _method(): pass
+            def _method():
+                raise NotImplementedError()
         self.assertRaises(ValueError, decorator, Bar._method)
 
     def test_oldstyle_class(self):
@@ -752,9 +758,8 @@ def test_simple(self):
             warnings.resetwarnings()
             try:
                 exec(CODE, globs, locs)
-            except TypeError: # pragma: no cover (tox runs coverage on Python 2)
-                if not PYTHON3:
-                    raise
+            except TypeError:
+                self.assertTrue(PYTHON3, "Must be Python 3")
             else:
                 if PYTHON3:
                     self.fail("Didn't raise TypeError")
@@ -1130,7 +1135,7 @@ def test_optimizations(self):
         from zope.interface.declarations import ClassProvidesBaseFallback
         try:
             import zope.interface._zope_interface_coptimizations
-        except ImportError: # pragma: no cover (pypy)
+        except ImportError:
             self.assertIs(self._getTargetClass(), ClassProvidesBaseFallback)
         else:
             self.assertIsNot(self._getTargetClass(), ClassProvidesBaseFallback)
@@ -1441,7 +1446,7 @@ def test_optimizations(self):
         from zope.interface.declarations import getObjectSpecification
         try:
             import zope.interface._zope_interface_coptimizations
-        except ImportError: # pragma: no cover (pypy)
+        except ImportError:
             self.assertIs(getObjectSpecification,
                           getObjectSpecificationFallback)
         else:
@@ -1538,7 +1543,7 @@ def test_optimizations(self):
         from zope.interface.declarations import providedBy
         try:
             import zope.interface._zope_interface_coptimizations
-        except ImportError: # pragma: no cover (pypy)
+        except ImportError:
             self.assertIs(providedBy, providedByFallback)
         else:
             self.assertIsNot(providedBy, providedByFallback)
@@ -1609,7 +1614,7 @@ def test_optimizations(self):
             ObjectSpecificationDescriptorFallback)
         try:
             import zope.interface._zope_interface_coptimizations
-        except ImportError: # pragma: no cover (pypy)
+        except ImportError:
             self.assertIs(self._getTargetClass(),
                           ObjectSpecificationDescriptorFallback)
         else:
diff --git a/src/zope/interface/tests/test_exceptions.py b/src/zope/interface/tests/test_exceptions.py
index 94009f6f..ae73f9cf 100644
--- a/src/zope/interface/tests/test_exceptions.py
+++ b/src/zope/interface/tests/test_exceptions.py
@@ -27,9 +27,8 @@ def _getTargetClass(self):
         from zope.interface.exceptions import DoesNotImplement
         return DoesNotImplement
 
-    def _makeOne(self, iface=None):
-        if iface is None:
-            iface = _makeIface()
+    def _makeOne(self):
+        iface = _makeIface()
         return self._getTargetClass()(iface)
 
     def test___str__(self):
@@ -45,9 +44,8 @@ def _getTargetClass(self):
         from zope.interface.exceptions import BrokenImplementation
         return BrokenImplementation
 
-    def _makeOne(self, iface=None, name='missing'):
-        if iface is None:
-            iface = _makeIface()
+    def _makeOne(self, name='missing'):
+        iface = _makeIface()
         return self._getTargetClass()(iface, name)
 
     def test___str__(self):
@@ -72,4 +70,3 @@ def test___str__(self):
         self.assertEqual(str(dni),
             'The implementation of aMethod violates its contract\n'
              '        because I said so.\n        ')
-
diff --git a/src/zope/interface/tests/test_interface.py b/src/zope/interface/tests/test_interface.py
index f14c76fe..58ed8084 100644
--- a/src/zope/interface/tests/test_interface.py
+++ b/src/zope/interface/tests/test_interface.py
@@ -16,6 +16,8 @@
 # pylint:disable=protected-access
 import unittest
 
+from zope.interface._compat import _skip_under_py3k
+
 _marker = object()
 
 
@@ -214,7 +216,7 @@ def test_optimizations(self):
         from zope.interface.interface import SpecificationBasePy
         try:
             import zope.interface._zope_interface_coptimizations
-        except ImportError: # pragma: no cover (pypy)
+        except ImportError:
             self.assertIs(self._getTargetClass(), SpecificationBasePy)
         else:
             self.assertIsNot(self._getTargetClass(), SpecificationBasePy)
@@ -291,7 +293,7 @@ def test_optimizations(self):
         from zope.interface.interface import InterfaceBasePy
         try:
             import zope.interface._zope_interface_coptimizations
-        except ImportError: # pragma: no cover (pypy)
+        except ImportError:
             self.assertIs(self._getTargetClass(), InterfaceBasePy)
         else:
             self.assertIsNot(self._getTargetClass(), InterfaceBasePy)
@@ -1677,21 +1679,22 @@ class I(Interface):
         self.assertEqual(I.__doc__, "")
         self.assertEqual(list(I), ['__doc__'])
 
+    @_skip_under_py3k
     def testIssue228(self):
         # Test for http://collector.zope.org/Zope3-dev/228
         # Old style classes don't have a '__class__' attribute
+        # No old style classes in Python 3, so the test becomes moot.
         import sys
-        if sys.version[0] < '3':
-            # No old style classes in Python 3, so the test becomes moot.
-            from zope.interface import Interface
 
-            class I(Interface):
-                "xxx"
+        from zope.interface import Interface
+
+        class I(Interface):
+            "xxx"
 
-            class OldStyle:
-                __providedBy__ = None
+        class OldStyle:
+            __providedBy__ = None
 
-            self.assertRaises(AttributeError, I.providedBy, OldStyle)
+        self.assertRaises(AttributeError, I.providedBy, OldStyle)
 
     def test_invariant_as_decorator(self):
         from zope.interface import Interface
diff --git a/src/zope/interface/tests/test_odd_declarations.py b/src/zope/interface/tests/test_odd_declarations.py
index 5480730b..46e7675f 100644
--- a/src/zope/interface/tests/test_odd_declarations.py
+++ b/src/zope/interface/tests/test_odd_declarations.py
@@ -257,9 +257,12 @@ class C(A, B):
         self.assertEqual(c.c, 1)
         c.c
 
-        import sys
-        if sys.version[0] == '2': # This test only makes sense under Python 2.x
+        try:
             from types import ClassType
+        except ImportError:
+            pass
+        else:
+            # This test only makes sense under Python 2.x
             assert not isinstance(C, (type, ClassType))
 
         self.assertIs(C.__class__.__class__, C.__class__)
diff --git a/src/zope/interface/verify.py b/src/zope/interface/verify.py
index 5a0e684a..098b484a 100644
--- a/src/zope/interface/verify.py
+++ b/src/zope/interface/verify.py
@@ -66,10 +66,10 @@ def _verify(iface, candidate, tentative=0, vtype=None):
             continue
 
         if isinstance(attr, FunctionType):
-            if sys.version[0] == '3' and isinstance(candidate, type):
+            if sys.version_info[0] >= 3 and isinstance(candidate, type):
                 # This is an "unbound method" in Python 3.
                 meth = fromFunction(attr, iface, name=name,
-                                    imlevel=1) # pragma: no cover
+                                    imlevel=1)
             else:
                 # Nope, just a normal function
                 meth = fromFunction(attr, iface, name=name)
@@ -85,7 +85,7 @@ def _verify(iface, candidate, tentative=0, vtype=None):
                 raise BrokenMethodImplementation(name, "Not a method")
             # sigh, it's callable, but we don't know how to introspect it, so
             # we have to give it a pass.
-            continue # pragma: no cover
+            continue
 
         # Make sure that the required and implemented method signatures are
         # the same.
diff --git a/tox.ini b/tox.ini
index 608fa008..3ed2be4b 100644
--- a/tox.ini
+++ b/tox.ini
@@ -4,9 +4,12 @@ envlist =
 
 [testenv]
 commands =
-    python setup.py -q test -q {posargs}
+    coverage run setup.py -q test -q {posargs}
 deps =
-    zope.event
+    .[test]
+    coverage
+setenv =
+   COVERAGE_FILE=.coverage.{envname}
 
 [testenv:py27-pure]
 setenv =
@@ -24,15 +27,15 @@ commands =
     {[testenv]commands}
 
 [testenv:coverage]
-usedevelop = true
-basepython =
-    python2.7
+setenv =
+  COVERAGE_FILE=.coverage
+skip_install = true
 commands =
-    coverage run setup.py -q test -q {posargs}
-    coverage report --skip-covered
-deps =
-    {[testenv]deps}
-    coverage
+    coverage erase
+    coverage combine
+    coverage report
+    coverage html
+    coverage xml
 
 [testenv:docs]
 basepython =