diff --git a/django/contrib/contenttypes/tests.py b/django/contrib/contenttypes/tests.py
index 756430f393..b194755b80 100644
--- a/django/contrib/contenttypes/tests.py
+++ b/django/contrib/contenttypes/tests.py
@@ -1,9 +1,10 @@
 from __future__ import unicode_literals
 
-from django.db import models
 from django.contrib.contenttypes.models import ContentType
 from django.contrib.contenttypes.views import shortcut
-from django.contrib.sites.models import Site, get_current_site
+from django.contrib.sites.models import get_current_site
+from django.core.apps import app_cache
+from django.db import models
 from django.http import HttpRequest, Http404
 from django.test import TestCase
 from django.test.utils import override_settings
@@ -54,11 +55,9 @@ class FooWithBrokenAbsoluteUrl(FooWithoutUrl):
 class ContentTypesTests(TestCase):
 
     def setUp(self):
-        self._old_installed = Site._meta.app_config.installed
         ContentType.objects.clear_cache()
 
     def tearDown(self):
-        Site._meta.app_config.installed = self._old_installed
         ContentType.objects.clear_cache()
 
     def test_lookup_cache(self):
@@ -223,15 +222,15 @@ class ContentTypesTests(TestCase):
         user_ct = ContentType.objects.get_for_model(FooWithUrl)
         obj = FooWithUrl.objects.create(name="john")
 
-        Site._meta.app_config.installed = True
-        response = shortcut(request, user_ct.id, obj.id)
-        self.assertEqual("http://%s/users/john/" % get_current_site(request).domain,
-                         response._headers.get("location")[1])
+        with app_cache._with_app('django.contrib.sites'):
+            response = shortcut(request, user_ct.id, obj.id)
+            self.assertEqual("http://%s/users/john/" % get_current_site(request).domain,
+                             response._headers.get("location")[1])
 
-        Site._meta.app_config.installed = False
-        response = shortcut(request, user_ct.id, obj.id)
-        self.assertEqual("http://Example.com/users/john/",
-                         response._headers.get("location")[1])
+        with app_cache._without_app('django.contrib.sites'):
+            response = shortcut(request, user_ct.id, obj.id)
+            self.assertEqual("http://Example.com/users/john/",
+                             response._headers.get("location")[1])
 
     def test_shortcut_view_without_get_absolute_url(self):
         """
diff --git a/django/contrib/gis/tests/geoapp/test_feeds.py b/django/contrib/gis/tests/geoapp/test_feeds.py
index 0817e65cb4..6f0d901a62 100644
--- a/django/contrib/gis/tests/geoapp/test_feeds.py
+++ b/django/contrib/gis/tests/geoapp/test_feeds.py
@@ -7,6 +7,7 @@ from django.conf import settings
 from django.contrib.sites.models import Site
 from django.contrib.gis.geos import HAS_GEOS
 from django.contrib.gis.tests.utils import HAS_SPATIAL_DB
+from django.core.apps import app_cache
 from django.test import TestCase
 
 if HAS_GEOS:
@@ -20,11 +21,10 @@ class GeoFeedTest(TestCase):
 
     def setUp(self):
         Site(id=settings.SITE_ID, domain="example.com", name="example.com").save()
-        self._old_installed = Site._meta.app_config.installed
-        Site._meta.app_config.installed = True
+        self._with_sites = app_cache._begin_with_app('django.contrib.sites')
 
     def tearDown(self):
-        Site._meta.app_config.installed = self._old_installed
+        app_cache._end_with_app(self._with_sites)
 
     def assertChildNodes(self, elem, expected):
         "Taken from syndication/tests.py."
diff --git a/django/contrib/gis/tests/geoapp/test_sitemaps.py b/django/contrib/gis/tests/geoapp/test_sitemaps.py
index ee9974ceaa..035d97af34 100644
--- a/django/contrib/gis/tests/geoapp/test_sitemaps.py
+++ b/django/contrib/gis/tests/geoapp/test_sitemaps.py
@@ -10,6 +10,7 @@ from django.conf import settings
 from django.contrib.gis.geos import HAS_GEOS
 from django.contrib.gis.tests.utils import HAS_SPATIAL_DB
 from django.contrib.sites.models import Site
+from django.core.apps import app_cache
 from django.test import TestCase
 from django.test.utils import IgnoreDeprecationWarningsMixin
 from django.utils._os import upath
@@ -26,11 +27,10 @@ class GeoSitemapTest(IgnoreDeprecationWarningsMixin, TestCase):
     def setUp(self):
         super(GeoSitemapTest, self).setUp()
         Site(id=settings.SITE_ID, domain="example.com", name="example.com").save()
-        self._old_installed = Site._meta.app_config.installed
-        Site._meta.app_config.installed = True
+        self._with_sites = app_cache._begin_with_app('django.contrib.sites')
 
     def tearDown(self):
-        Site._meta.app_config.installed = self._old_installed
+        app_cache._end_with_app(self._with_sites)
         super(GeoSitemapTest, self).tearDown()
 
     def assertChildNodes(self, elem, expected):
diff --git a/django/contrib/sitemaps/tests/base.py b/django/contrib/sitemaps/tests/base.py
index 98abb3dca4..ecddcc737b 100644
--- a/django/contrib/sitemaps/tests/base.py
+++ b/django/contrib/sitemaps/tests/base.py
@@ -25,10 +25,6 @@ class SitemapTestsBase(TestCase):
 
     def setUp(self):
         self.base_url = '%s://%s' % (self.protocol, self.domain)
-        self._old_installed = Site._meta.app_config.installed
         cache.clear()
         # Create an object for sitemap content.
         TestModel.objects.create(name='Test Object')
-
-    def tearDown(self):
-        Site._meta.app_config.installed = self._old_installed
diff --git a/django/contrib/sitemaps/tests/test_http.py b/django/contrib/sitemaps/tests/test_http.py
index 61be8c842a..ed61c9950b 100644
--- a/django/contrib/sitemaps/tests/test_http.py
+++ b/django/contrib/sitemaps/tests/test_http.py
@@ -7,6 +7,7 @@ from unittest import skipUnless
 from django.conf import settings
 from django.contrib.sitemaps import Sitemap, GenericSitemap
 from django.contrib.sites.models import Site
+from django.core.apps import app_cache
 from django.core.exceptions import ImproperlyConfigured
 from django.test.utils import override_settings
 from django.utils.formats import localize
@@ -108,15 +109,14 @@ class HTTPSitemapTests(SitemapTestsBase):
     def test_requestsite_sitemap(self):
         # Make sure hitting the flatpages sitemap without the sites framework
         # installed doesn't raise an exception.
-        # Reset by SitemapTestsBase.tearDown().
-        Site._meta.app_config.installed = False
-        response = self.client.get('/simple/sitemap.xml')
-        expected_content = """<?xml version="1.0" encoding="UTF-8"?>
+        with app_cache._without_app('django.contrib.sites'):
+            response = self.client.get('/simple/sitemap.xml')
+            expected_content = """<?xml version="1.0" encoding="UTF-8"?>
 <urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
 <url><loc>http://testserver/location/</loc><lastmod>%s</lastmod><changefreq>never</changefreq><priority>0.5</priority></url>
 </urlset>
 """ % date.today()
-        self.assertXMLEqual(response.content.decode('utf-8'), expected_content)
+            self.assertXMLEqual(response.content.decode('utf-8'), expected_content)
 
     @skipUnless("django.contrib.sites" in settings.INSTALLED_APPS,
                 "django.contrib.sites app not installed.")
@@ -134,9 +134,8 @@ class HTTPSitemapTests(SitemapTestsBase):
         Sitemap.get_urls if Site objects exists, but the sites framework is not
         actually installed.
         """
-        # Reset by SitemapTestsBase.tearDown().
-        Site._meta.app_config.installed = False
-        self.assertRaises(ImproperlyConfigured, Sitemap().get_urls)
+        with app_cache._without_app('django.contrib.sites'):
+            self.assertRaises(ImproperlyConfigured, Sitemap().get_urls)
 
     def test_sitemap_item(self):
         """
diff --git a/django/contrib/sites/tests.py b/django/contrib/sites/tests.py
index 2be28a1429..8d76fd0d02 100644
--- a/django/contrib/sites/tests.py
+++ b/django/contrib/sites/tests.py
@@ -2,6 +2,7 @@ from __future__ import unicode_literals
 
 from django.conf import settings
 from django.contrib.sites.models import Site, RequestSite, get_current_site
+from django.core.apps import app_cache
 from django.core.exceptions import ObjectDoesNotExist, ValidationError
 from django.http import HttpRequest
 from django.test import TestCase
@@ -12,11 +13,10 @@ class SitesFrameworkTests(TestCase):
 
     def setUp(self):
         Site(id=settings.SITE_ID, domain="example.com", name="example.com").save()
-        self._old_installed = Site._meta.app_config.installed
-        Site._meta.app_config.installed = True
+        self._with_sites = app_cache._begin_with_app('django.contrib.sites')
 
     def tearDown(self):
-        Site._meta.app_config.installed = self._old_installed
+        app_cache._end_with_app(self._with_sites)
 
     def test_save_another(self):
         # Regression for #17415
@@ -67,10 +67,10 @@ class SitesFrameworkTests(TestCase):
         self.assertRaises(ObjectDoesNotExist, get_current_site, request)
 
         # A RequestSite is returned if the sites framework is not installed
-        Site._meta.app_config.installed = False
-        site = get_current_site(request)
-        self.assertTrue(isinstance(site, RequestSite))
-        self.assertEqual(site.name, "example.com")
+        with app_cache._without_app('django.contrib.sites'):
+            site = get_current_site(request)
+            self.assertTrue(isinstance(site, RequestSite))
+            self.assertEqual(site.name, "example.com")
 
     def test_domain_name_with_whitespaces(self):
         # Regression for #17320
diff --git a/django/core/apps/base.py b/django/core/apps/base.py
index 5991029c09..332a066bcb 100644
--- a/django/core/apps/base.py
+++ b/django/core/apps/base.py
@@ -30,14 +30,10 @@ class AppConfig(object):
         # Populated by calls to AppCache.register_model().
         self.models = OrderedDict()
 
-        # Whether the app is in INSTALLED_APPS or was automatically created
-        # when one of its models was imported.
-        self.installed = app_module is not None
-
         # Filesystem path to the application directory eg.
         # u'/usr/lib/python2.7/dist-packages/django/contrib/admin'.
         # This is a unicode object on Python 2 and a str on Python 3.
-        self.path = upath(app_module.__path__[0]) if app_module is not None else None
+        self.path = upath(app_module.__path__[0])
 
     def __repr__(self):
         return '<AppConfig: %s>' % self.label
diff --git a/django/core/apps/cache.py b/django/core/apps/cache.py
index 69f2f2c974..fb7bb6172e 100644
--- a/django/core/apps/cache.py
+++ b/django/core/apps/cache.py
@@ -1,6 +1,7 @@
 "Utilities for loading models and the modules that contain them."
 
 from collections import defaultdict, OrderedDict
+from contextlib import contextmanager
 from importlib import import_module
 import os
 import sys
@@ -120,8 +121,7 @@ class AppCache(object):
         finally:
             self.nesting_level -= 1
 
-        app_config = AppConfig(
-            name=app_name, app_module=app_module, models_module=models_module)
+        app_config = AppConfig(app_name, app_module, models_module)
         app_config.models = self.all_models[app_config.label]
         self.app_configs[app_config.label] = app_config
 
@@ -257,7 +257,7 @@ class AppCache(object):
         self.populate()
         if only_installed:
             app_config = self.app_configs.get(app_label)
-            if app_config is None or not app_config.installed:
+            if app_config is None:
                 return None
             if (self.available_apps is not None
                     and app_config.name not in self.available_apps):
@@ -304,6 +304,63 @@ class AppCache(object):
     def unset_available_apps(self):
         self.available_apps = None
 
+    ### DANGEROUS METHODS ### (only used to preserve existing tests)
+
+    def _begin_with_app(self, app_name):
+        # Returns an opaque value that can be passed to _end_with_app().
+        app_module = import_module(app_name)
+        models_module = import_module('%s.models' % app_name)
+        app_config = AppConfig(app_name, app_module, models_module)
+        if app_config.label in self.app_configs:
+            return None
+        else:
+            app_config.models = self.all_models[app_config.label]
+            self.app_configs[app_config.label] = app_config
+            return app_config
+
+    def _end_with_app(self, app_config):
+        if app_config is not None:
+            del self.app_configs[app_config.label]
+
+    @contextmanager
+    def _with_app(self, app_name):
+        app_config = self._begin_with_app(app_name)
+        try:
+            yield
+        finally:
+            self._end_with_app(app_config)
+
+    def _begin_without_app(self, app_name):
+        # Returns an opaque value that can be passed to _end_without_app().
+        return self.app_configs.pop(app_name.rpartition(".")[2], None)
+
+    def _end_without_app(self, app_config):
+        if app_config is not None:
+            self.app_configs[app_config.label] = app_config
+
+    @contextmanager
+    def _without_app(self, app_name):
+        app_config = self._begin_without_app(app_name)
+        try:
+            yield
+        finally:
+            self._end_without_app(app_config)
+
+    def _begin_empty(self):
+        app_configs, self.app_configs = self.app_configs, OrderedDict()
+        return app_configs
+
+    def _end_empty(self, app_configs):
+        self.app_configs = app_configs
+
+    @contextmanager
+    def _empty(self):
+        app_configs = self._begin_empty()
+        try:
+            yield
+        finally:
+            self._end_empty(app_configs)
+
     ### DEPRECATED METHODS GO BELOW THIS LINE ###
 
     def get_app(self, app_label):
diff --git a/django/db/models/options.py b/django/db/models/options.py
index cd68d5eae9..f7be089539 100644
--- a/django/db/models/options.py
+++ b/django/db/models/options.py
@@ -94,11 +94,11 @@ class Options(object):
     @property
     def app_config(self):
         # Don't go through get_app_config to avoid triggering populate().
-        return self.app_cache.app_configs[self.app_label]
+        return self.app_cache.app_configs.get(self.app_label)
 
     @property
     def installed(self):
-        return self.app_config.installed
+        return self.app_config is not None
 
     def contribute_to_class(self, cls, name):
         from django.db import connection
diff --git a/tests/admin_docs/tests.py b/tests/admin_docs/tests.py
index 047bf920a2..66c125490f 100644
--- a/tests/admin_docs/tests.py
+++ b/tests/admin_docs/tests.py
@@ -4,6 +4,7 @@ from django.conf import settings
 from django.contrib.sites.models import Site
 from django.contrib.admindocs import utils
 from django.contrib.auth.models import User
+from django.core.apps import app_cache
 from django.core.urlresolvers import reverse
 from django.test import TestCase
 from django.test.utils import override_settings
@@ -13,27 +14,18 @@ class MiscTests(TestCase):
     urls = 'admin_docs.urls'
 
     def setUp(self):
-        self._old_installed = Site._meta.app_config.installed
         User.objects.create_superuser('super', None, 'secret')
         self.client.login(username='super', password='secret')
 
-    def tearDown(self):
-        Site._meta.app_config.installed = self._old_installed
-
-    @override_settings(
-        SITE_ID=None,
-        INSTALLED_APPS=[app for app in settings.INSTALLED_APPS
-                        if app != 'django.contrib.sites'],
-    )
     def test_no_sites_framework(self):
         """
         Without the sites framework, should not access SITE_ID or Site
         objects. Deleting settings is fine here as UserSettingsHolder is used.
         """
-        Site._meta.app_config.installed = False
-        Site.objects.all().delete()
-        del settings.SITE_ID
-        self.client.get('/admindocs/views/')  # should not raise
+        with self.settings(SITE_ID=None), app_cache._without_app('django.contrib.sites'):
+            Site.objects.all().delete()
+            del settings.SITE_ID
+            self.client.get('/admindocs/views/')  # should not raise
 
 
 @override_settings(PASSWORD_HASHERS=('django.contrib.auth.hashers.SHA1PasswordHasher',))