diff --git a/tests/base.py b/tests/base.py
index f210591..2e4e9a5 100755
--- a/tests/base.py
+++ b/tests/base.py
@@ -2148,6 +2148,8 @@
         # Make sure we set up an RSA key for the project so that we
         # don't spend time generating one:
 
+        if isinstance(project, dict):
+            project = list(project.keys())[0]
         key_root = os.path.join(self.state_root, 'keys')
         if not os.path.isdir(key_root):
             os.mkdir(key_root, 0o700)
diff --git a/tests/fixtures/config/shadow/git/local-config/playbooks/base.yaml b/tests/fixtures/config/shadow/git/local-config/playbooks/base.yaml
new file mode 100644
index 0000000..f679dce
--- /dev/null
+++ b/tests/fixtures/config/shadow/git/local-config/playbooks/base.yaml
@@ -0,0 +1,2 @@
+- hosts: all
+  tasks: []
diff --git a/tests/fixtures/config/shadow/git/local-config/playbooks/test2.yaml b/tests/fixtures/config/shadow/git/local-config/playbooks/test2.yaml
new file mode 100644
index 0000000..f679dce
--- /dev/null
+++ b/tests/fixtures/config/shadow/git/local-config/playbooks/test2.yaml
@@ -0,0 +1,2 @@
+- hosts: all
+  tasks: []
diff --git a/tests/fixtures/config/shadow/git/local-config/zuul.yaml b/tests/fixtures/config/shadow/git/local-config/zuul.yaml
new file mode 100644
index 0000000..756e843
--- /dev/null
+++ b/tests/fixtures/config/shadow/git/local-config/zuul.yaml
@@ -0,0 +1,25 @@
+- pipeline:
+    name: check
+    manager: independent
+    trigger:
+      gerrit:
+        - event: patchset-created
+    success:
+      gerrit:
+        verified: 1
+    failure:
+      gerrit:
+        verified: -1
+
+- job:
+    name: base
+
+- job:
+    name: test2
+
+- project:
+    name: org/project
+    check:
+      jobs:
+        - test1
+        - test2
diff --git a/tests/fixtures/config/shadow/git/org_project/README b/tests/fixtures/config/shadow/git/org_project/README
new file mode 100644
index 0000000..9daeafb
--- /dev/null
+++ b/tests/fixtures/config/shadow/git/org_project/README
@@ -0,0 +1 @@
+test
diff --git a/tests/fixtures/config/shadow/git/stdlib/.zuul.yaml b/tests/fixtures/config/shadow/git/stdlib/.zuul.yaml
new file mode 100644
index 0000000..6a6f9c9
--- /dev/null
+++ b/tests/fixtures/config/shadow/git/stdlib/.zuul.yaml
@@ -0,0 +1,10 @@
+- job:
+    name: base
+
+- job:
+    name: test1
+    parent: base
+
+- job:
+    name: test2
+    parent: base
diff --git a/tests/fixtures/config/shadow/git/stdlib/README b/tests/fixtures/config/shadow/git/stdlib/README
new file mode 100644
index 0000000..9daeafb
--- /dev/null
+++ b/tests/fixtures/config/shadow/git/stdlib/README
@@ -0,0 +1 @@
+test
diff --git a/tests/fixtures/config/shadow/git/stdlib/playbooks/base.yaml b/tests/fixtures/config/shadow/git/stdlib/playbooks/base.yaml
new file mode 100644
index 0000000..f679dce
--- /dev/null
+++ b/tests/fixtures/config/shadow/git/stdlib/playbooks/base.yaml
@@ -0,0 +1,2 @@
+- hosts: all
+  tasks: []
diff --git a/tests/fixtures/config/shadow/git/stdlib/playbooks/test1.yaml b/tests/fixtures/config/shadow/git/stdlib/playbooks/test1.yaml
new file mode 100644
index 0000000..f679dce
--- /dev/null
+++ b/tests/fixtures/config/shadow/git/stdlib/playbooks/test1.yaml
@@ -0,0 +1,2 @@
+- hosts: all
+  tasks: []
diff --git a/tests/fixtures/config/shadow/git/stdlib/playbooks/test2.yaml b/tests/fixtures/config/shadow/git/stdlib/playbooks/test2.yaml
new file mode 100644
index 0000000..f679dce
--- /dev/null
+++ b/tests/fixtures/config/shadow/git/stdlib/playbooks/test2.yaml
@@ -0,0 +1,2 @@
+- hosts: all
+  tasks: []
diff --git a/tests/fixtures/config/shadow/main.yaml b/tests/fixtures/config/shadow/main.yaml
new file mode 100644
index 0000000..f148a84
--- /dev/null
+++ b/tests/fixtures/config/shadow/main.yaml
@@ -0,0 +1,10 @@
+- tenant:
+    name: tenant-one
+    source:
+      gerrit:
+        config-projects:
+          - local-config
+        untrusted-projects:
+          - stdlib:
+              shadow: local-config
+          - org/project
diff --git a/tests/unit/test_model.py b/tests/unit/test_model.py
index 3ab3305..7fe101e 100644
--- a/tests/unit/test_model.py
+++ b/tests/unit/test_model.py
@@ -40,7 +40,7 @@
         self.source = Dummy(canonical_hostname='git.example.com',
                             connection=self.connection)
         self.tenant = model.Tenant('tenant')
-        self.layout = model.Layout()
+        self.layout = model.Layout(self.tenant)
         self.project = model.Project('project', self.source)
         self.tpc = model.TenantProjectConfig(self.project)
         self.tenant.addUntrustedProject(self.tpc)
@@ -59,7 +59,7 @@
     @property
     def job(self):
         tenant = model.Tenant('tenant')
-        layout = model.Layout()
+        layout = model.Layout(tenant)
         job = configloader.JobParser.fromYaml(tenant, layout, {
             '_source_context': self.context,
             '_start_mark': self.start_mark,
@@ -170,7 +170,7 @@
     def test_job_inheritance_configloader(self):
         # TODO(jeblair): move this to a configloader test
         tenant = model.Tenant('tenant')
-        layout = model.Layout()
+        layout = model.Layout(tenant)
 
         pipeline = model.Pipeline('gate', layout)
         layout.addPipeline(pipeline)
@@ -333,8 +333,8 @@
                           'playbooks/base'])
 
     def test_job_auth_inheritance(self):
-        tenant = model.Tenant('tenant')
-        layout = model.Layout()
+        tenant = self.tenant
+        layout = self.layout
 
         conf = yaml.safe_load('''
 - secret:
@@ -359,7 +359,7 @@
         secret = configloader.SecretParser.fromYaml(layout, conf)
         layout.addSecret(secret)
 
-        base = configloader.JobParser.fromYaml(tenant, layout, {
+        base = configloader.JobParser.fromYaml(self.tenant, self.layout, {
             '_source_context': self.context,
             '_start_mark': self.start_mark,
             'name': 'base',
@@ -443,7 +443,7 @@
 
     def test_job_inheritance_job_tree(self):
         tenant = model.Tenant('tenant')
-        layout = model.Layout()
+        layout = model.Layout(tenant)
         tpc = model.TenantProjectConfig(self.project)
         tenant.addUntrustedProject(tpc)
 
@@ -520,7 +520,7 @@
 
     def test_inheritance_keeps_matchers(self):
         tenant = model.Tenant('tenant')
-        layout = model.Layout()
+        layout = model.Layout(tenant)
 
         pipeline = model.Pipeline('gate', layout)
         layout.addPipeline(pipeline)
@@ -571,11 +571,13 @@
         self.assertEqual([], item.getJobs())
 
     def test_job_source_project(self):
-        tenant = model.Tenant('tenant')
-        layout = model.Layout()
+        tenant = self.tenant
+        layout = self.layout
         base_project = model.Project('base_project', self.source)
         base_context = model.SourceContext(base_project, 'master',
                                            'test', True)
+        tpc = model.TenantProjectConfig(base_project)
+        tenant.addUntrustedProject(tpc)
 
         base = configloader.JobParser.fromYaml(tenant, layout, {
             '_source_context': base_context,
@@ -587,6 +589,8 @@
         other_project = model.Project('other_project', self.source)
         other_context = model.SourceContext(other_project, 'master',
                                             'test', True)
+        tpc = model.TenantProjectConfig(other_project)
+        tenant.addUntrustedProject(tpc)
         base2 = configloader.JobParser.fromYaml(tenant, layout, {
             '_source_context': other_context,
             '_start_mark': self.start_mark,
diff --git a/tests/unit/test_v3.py b/tests/unit/test_v3.py
index 327f745..112f48c 100644
--- a/tests/unit/test_v3.py
+++ b/tests/unit/test_v3.py
@@ -630,3 +630,17 @@
         self.assertHistory([
             dict(name='project-test', result='SUCCESS', changes='1,1 2,1'),
         ])
+
+
+class TestShadow(ZuulTestCase):
+    tenant_config_file = 'config/shadow/main.yaml'
+
+    def test_shadow(self):
+        # Test that a repo is allowed to shadow another's job definitions.
+        A = self.fake_gerrit.addFakeChange('org/project', 'master', 'A')
+        self.fake_gerrit.addEvent(A.getPatchsetCreatedEvent(1))
+        self.waitUntilSettled()
+        self.assertHistory([
+            dict(name='test1', result='SUCCESS', changes='1,1'),
+            dict(name='test2', result='SUCCESS', changes='1,1'),
+        ])
diff --git a/zuul/configloader.py b/zuul/configloader.py
index ccf35da..735fe38 100644
--- a/zuul/configloader.py
+++ b/zuul/configloader.py
@@ -887,6 +887,7 @@
     project_dict = {str: {
         'include': to_list(classes),
         'exclude': to_list(classes),
+        'shadow': to_list(str),
     }}
 
     project = vs.Any(str, project_dict)
@@ -940,6 +941,10 @@
             tenant.addConfigProject(tpc)
         for tpc in untrusted_tpcs:
             tenant.addUntrustedProject(tpc)
+
+        for tpc in config_tpcs + untrusted_tpcs:
+            TenantParser._resolveShadowProjects(tenant, tpc)
+
         tenant.config_projects_config, tenant.untrusted_projects_config = \
             TenantParser._loadTenantInRepoLayouts(merger, connections,
                                                   tenant.config_projects,
@@ -954,6 +959,13 @@
         return tenant
 
     @staticmethod
+    def _resolveShadowProjects(tenant, tpc):
+        shadow_projects = []
+        for sp in tpc.shadow_projects:
+            shadow_projects.append(tenant.getProject(sp)[1])
+        tpc.shadow_projects = frozenset(shadow_projects)
+
+    @staticmethod
     def _loadProjectKeys(project_key_dir, connection_name, project):
         project.private_key_file = (
             os.path.join(project_key_dir, connection_name,
@@ -1008,9 +1020,11 @@
             # Return a project object whether conf is a dict or a str
             project = source.getProject(conf)
             project_include = current_include
+            shadow_projects = []
         else:
             project_name = list(conf.keys())[0]
             project = source.getProject(project_name)
+            shadow_projects = as_list(conf[project_name].get('shadow', []))
 
             project_include = frozenset(
                 as_list(conf[project_name].get('include', [])))
@@ -1023,6 +1037,7 @@
 
         tenant_project_config = model.TenantProjectConfig(project)
         tenant_project_config.load_classes = frozenset(project_include)
+        tenant_project_config.shadow_projects = shadow_projects
 
         return tenant_project_config
 
@@ -1240,7 +1255,11 @@
                 continue
             with configuration_exceptions('job', config_job):
                 job = JobParser.fromYaml(tenant, layout, config_job)
-                layout.addJob(job)
+                added = layout.addJob(job)
+                if not added:
+                    TenantParser.log.debug(
+                        "Skipped adding job %s which shadows an existing job" %
+                        (job,))
 
         if not skip_semaphores:
             for config_semaphore in data.semaphores:
@@ -1279,13 +1298,11 @@
 
     @staticmethod
     def _parseLayout(base, tenant, data, scheduler, connections):
-        layout = model.Layout()
+        layout = model.Layout(tenant)
 
         TenantParser._parseLayoutItems(layout, tenant, data,
                                        scheduler, connections)
 
-        layout.tenant = tenant
-
         for pipeline in layout.pipelines.values():
             pipeline.manager._postConfig(layout)
 
@@ -1401,7 +1418,7 @@
         for project in tenant.untrusted_projects:
             self._loadDynamicProjectData(config, project, files, False)
 
-        layout = model.Layout()
+        layout = model.Layout(tenant)
         # NOTE: the actual pipeline objects (complete with queues and
         # enqueued items) are copied by reference here.  This allows
         # our shadow dynamic configuration to continue to interact
diff --git a/zuul/model.py b/zuul/model.py
index d233415..4744bbe 100644
--- a/zuul/model.py
+++ b/zuul/model.py
@@ -2009,6 +2009,7 @@
     def __init__(self, project):
         self.project = project
         self.load_classes = set()
+        self.shadow_projects = set()
 
 
 class ProjectConfig(object):
@@ -2132,8 +2133,8 @@
 class Layout(object):
     """Holds all of the Pipelines."""
 
-    def __init__(self):
-        self.tenant = None
+    def __init__(self, tenant):
+        self.tenant = tenant
         self.project_configs = {}
         self.project_templates = {}
         self.pipelines = OrderedDict()
@@ -2162,6 +2163,18 @@
         prior_jobs = [j for j in self.getJobs(job.name) if
                       j.source_context.project !=
                       job.source_context.project]
+        # Unless the repo is permitted to shadow another.  If so, and
+        # the job we are adding is from a repo that is permitted to
+        # shadow the one with the older jobs, skip adding this job.
+        job_project = job.source_context.project
+        job_tpc = self.tenant.project_configs[job_project.canonical_name]
+        skip_add = False
+        for prior_job in prior_jobs[:]:
+            prior_project = prior_job.source_context.project
+            if prior_project in job_tpc.shadow_projects:
+                prior_jobs.remove(prior_job)
+                skip_add = True
+
         if prior_jobs:
             raise Exception("Job %s in %s is not permitted to shadow "
                             "job %s in %s" % (
@@ -2169,11 +2182,13 @@
                                 job.source_context.project,
                                 prior_jobs[0],
                                 prior_jobs[0].source_context.project))
-
+        if skip_add:
+            return False
         if job.name in self.jobs:
             self.jobs[job.name].append(job)
         else:
             self.jobs[job.name] = [job]
+        return True
 
     def addNodeSet(self, nodeset):
         if nodeset.name in self.nodesets: