diff --git a/news/5416.bugfix b/news/5416.bugfix
new file mode 100644
index 00000000000..2f7707e0ce6
--- /dev/null
+++ b/news/5416.bugfix
@@ -0,0 +1 @@
+Start refusing to install packages with non PEP-518 compliant pyproject.toml
diff --git a/news/5512.bugfix b/news/5512.bugfix
new file mode 100644
index 00000000000..2f7707e0ce6
--- /dev/null
+++ b/news/5512.bugfix
@@ -0,0 +1 @@
+Start refusing to install packages with non PEP-518 compliant pyproject.toml
diff --git a/src/pip/_internal/operations/prepare.py b/src/pip/_internal/operations/prepare.py
index ff9b1e6a1df..3ad721f019a 100644
--- a/src/pip/_internal/operations/prepare.py
+++ b/src/pip/_internal/operations/prepare.py
@@ -95,31 +95,33 @@ def dist(self, finder):
     def prep_for_dist(self, finder, build_isolation):
         # Before calling "setup.py egg_info", we need to set-up the build
         # environment.
-        build_requirements, isolate = self.req.get_pep_518_info()
-        should_isolate = build_isolation and isolate
-
-        minimum_requirements = ('setuptools', 'wheel')
-        missing_requirements = set(minimum_requirements) - set(
-            pkg_resources.Requirement(r).key
-            for r in build_requirements
-        )
-        if missing_requirements:
-            def format_reqs(rs):
-                return ' and '.join(map(repr, sorted(rs)))
-            logger.warning(
-                "Missing build time requirements in pyproject.toml for %s: "
-                "%s.", self.req, format_reqs(missing_requirements)
-            )
-            logger.warning(
-                "This version of pip does not implement PEP 517 so it cannot "
-                "build a wheel without %s.", format_reqs(minimum_requirements)
-            )
+        build_requirements = self.req.get_pep_518_info()
+        should_isolate = build_isolation and build_requirements is not None
 
         if should_isolate:
+            # Haven't implemented PEP 517 yet, so spew a warning about it if
+            # build-requirements don't include setuptools and wheel.
+            missing_requirements = {'setuptools', 'wheel'} - {
+                pkg_resources.Requirement(r).key for r in build_requirements
+            }
+            if missing_requirements:
+                logger.warning(
+                    "Missing build requirements in pyproject.toml for %s.",
+                    self.req,
+                )
+                logger.warning(
+                    "This version of pip does not implement PEP 517 so it "
+                    "cannot build a wheel without %s.",
+                    " and ".join(map(repr, sorted(missing_requirements)))
+                )
+
+            # Isolate in a BuildEnvironment and install the build-time
+            # requirements.
             self.req.build_env = BuildEnvironment()
             self.req.build_env.install_requirements(
                 finder, build_requirements,
-                "Installing build dependencies")
+                "Installing build dependencies"
+            )
 
         self.req.run_egg_info()
         self.req.assert_source_matches_version()
diff --git a/src/pip/_internal/req/req_install.py b/src/pip/_internal/req/req_install.py
index 7846f3c169f..bd6d4e9fe95 100644
--- a/src/pip/_internal/req/req_install.py
+++ b/src/pip/_internal/req/req_install.py
@@ -33,7 +33,9 @@
     PIP_DELETE_MARKER_FILENAME, running_under_virtualenv,
 )
 from pip._internal.req.req_uninstall import UninstallPathSet
-from pip._internal.utils.deprecation import RemovedInPip11Warning
+from pip._internal.utils.deprecation import (
+    RemovedInPip11Warning, RemovedInPip12Warning,
+)
 from pip._internal.utils.hashes import Hashes
 from pip._internal.utils.logging import indent_log
 from pip._internal.utils.misc import (
@@ -560,20 +562,51 @@ def pyproject_toml(self):
         return pp_toml
 
     def get_pep_518_info(self):
-        """Get a list of the packages required to build the project, if any,
-        and a flag indicating whether pyproject.toml is present, indicating
-        that the build should be isolated.
+        """Get PEP 518 build-time requirements.
 
-        Build requirements can be specified in a pyproject.toml, as described
-        in PEP 518. If this file exists but doesn't specify build
-        requirements, pip will default to installing setuptools and wheel.
+        Returns the list of the packages required to build the project,
+        specified as per PEP 518 within the package. If `pyproject.toml` is not
+        present, returns None to signify not using the same.
         """
-        if os.path.isfile(self.pyproject_toml):
-            with io.open(self.pyproject_toml, encoding="utf-8") as f:
-                pp_toml = pytoml.load(f)
-            build_sys = pp_toml.get('build-system', {})
-            return (build_sys.get('requires', ['setuptools', 'wheel']), True)
-        return (['setuptools', 'wheel'], False)
+        if not os.path.isfile(self.pyproject_toml):
+            return None
+
+        with io.open(self.pyproject_toml, encoding="utf-8") as f:
+            pp_toml = pytoml.load(f)
+
+        # Extract the build requirements
+        requires = pp_toml.get("build-system", {}).get("requires", None)
+
+        template = (
+            "%s does not comply with PEP 518 since pyproject.toml "
+            "does not contain a valid '[build-system].requires' key: %s"
+        )
+
+        if requires is None:
+            logging.warn(template, self, "it is missing.")
+            warnings.warn(
+                "Future versions of pip will reject packages with "
+                "pyproject.toml files that do not comply with PEP 518.",
+                RemovedInPip12Warning,
+            )
+
+            # NOTE: Currently allowing projects to skip this key so that they
+            #       can transition to a PEP 518 compliant pyproject.toml or
+            #       push to update the PEP.
+            # Come pip 19.0, bring this to compliance with PEP 518.
+            return None
+        else:
+            # Error out if it's not a list of strings
+            is_list_of_str = isinstance(requires, list) and all(
+                isinstance(req, six.string_types) for req in requires
+            )
+            if not is_list_of_str:
+                raise InstallationError(
+                    template % (self, "it is not a list of strings.")
+                )
+
+        # If control flow reaches here, we're good to go.
+        return requires
 
     def run_egg_info(self):
         assert self.source_dir
diff --git a/tests/data/src/pep518_invalid_requires/MANIFEST.in b/tests/data/src/pep518_invalid_requires/MANIFEST.in
new file mode 100644
index 00000000000..bec201fc83b
--- /dev/null
+++ b/tests/data/src/pep518_invalid_requires/MANIFEST.in
@@ -0,0 +1 @@
+include pyproject.toml
diff --git a/tests/data/src/pep518_invalid_requires/pep518.py b/tests/data/src/pep518_invalid_requires/pep518.py
new file mode 100644
index 00000000000..7986d11379a
--- /dev/null
+++ b/tests/data/src/pep518_invalid_requires/pep518.py
@@ -0,0 +1 @@
+#dummy
diff --git a/tests/data/src/pep518_invalid_requires/pyproject.toml b/tests/data/src/pep518_invalid_requires/pyproject.toml
new file mode 100644
index 00000000000..17bc681d654
--- /dev/null
+++ b/tests/data/src/pep518_invalid_requires/pyproject.toml
@@ -0,0 +1,2 @@
+[build-system]
+requires = [1, 2, 3]  # not a list of strings
diff --git a/tests/data/src/pep518_invalid_requires/setup.py b/tests/data/src/pep518_invalid_requires/setup.py
new file mode 100644
index 00000000000..e8a92da312a
--- /dev/null
+++ b/tests/data/src/pep518_invalid_requires/setup.py
@@ -0,0 +1,8 @@
+#!/usr/bin/env python
+from setuptools import setup
+
+setup(
+    name='pep518_invalid_requires',
+    version='1.0.0',
+    py_modules=['pep518'],
+)
diff --git a/tests/data/src/pep518_missing_requires/MANIFEST.in b/tests/data/src/pep518_missing_requires/MANIFEST.in
new file mode 100644
index 00000000000..bec201fc83b
--- /dev/null
+++ b/tests/data/src/pep518_missing_requires/MANIFEST.in
@@ -0,0 +1 @@
+include pyproject.toml
diff --git a/tests/data/src/pep518_missing_requires/pep518.py b/tests/data/src/pep518_missing_requires/pep518.py
new file mode 100644
index 00000000000..7986d11379a
--- /dev/null
+++ b/tests/data/src/pep518_missing_requires/pep518.py
@@ -0,0 +1 @@
+#dummy
diff --git a/tests/data/src/pep518_missing_requires/pyproject.toml b/tests/data/src/pep518_missing_requires/pyproject.toml
new file mode 100644
index 00000000000..e69de29bb2d
diff --git a/tests/data/src/pep518_missing_requires/setup.py b/tests/data/src/pep518_missing_requires/setup.py
new file mode 100644
index 00000000000..cbc5d28af04
--- /dev/null
+++ b/tests/data/src/pep518_missing_requires/setup.py
@@ -0,0 +1,8 @@
+#!/usr/bin/env python
+from setuptools import setup
+
+setup(
+    name='pep518_missing_requires',
+    version='1.0.0',
+    py_modules=['pep518'],
+)
diff --git a/tests/functional/test_install.py b/tests/functional/test_install.py
index 385db77ae34..b1025190706 100644
--- a/tests/functional/test_install.py
+++ b/tests/functional/test_install.py
@@ -37,6 +37,28 @@ def test_pep518_uses_build_env(script, data, common_wheels, command, variant):
     )
 
 
+def test_pep518_refuses_invalid_requires(script, data, common_wheels):
+    result = script.pip(
+        'install', '-f', common_wheels,
+        data.src.join("pep518_invalid_requires"),
+        expect_error=True
+    )
+    assert result.returncode == 1
+    assert "does not comply with PEP 518" in result.stderr
+
+
+def test_pep518_allows_but_warns_missing_requires(script, data, common_wheels):
+    result = script.pip(
+        'install', '-f', common_wheels,
+        data.src.join("pep518_missing_requires"),
+        expect_stderr=True
+    )
+    assert "does not comply with PEP 518" in result.stderr
+    assert "DEPRECATION" in result.stderr
+    assert result.returncode == 0
+    assert result.files_created
+
+
 def test_pep518_with_user_pip(script, virtualenv, pip_src,
                               data, common_wheels):
     virtualenv.system_site_packages = True