diff --git a/.gitignore b/.gitignore
index f523f52..ff722b0 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,3 +2,4 @@ build
 *~
 ognibuild.egg-info
 dist
+__pycache__
diff --git a/ognibuild/session/__init__.py b/ognibuild/session/__init__.py
new file mode 100644
index 0000000..6917def
--- /dev/null
+++ b/ognibuild/session/__init__.py
@@ -0,0 +1,66 @@
+#!/usr/bin/python
+# Copyright (C) 2020 Jelmer Vernooij <jelmer@jelmer.uk>
+# encoding: utf-8
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+
+from typing import Optional, List, Dict
+
+
+class Session(object):
+
+    def __enter__(self) -> 'Session':
+        return self
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        return False
+
+    def chdir(self, cwd: str) -> None:
+        raise NotImplementedError(self.chdir)
+
+    @property
+    def location(self) -> str:
+        raise NotImplementedError(self.location)
+
+    def check_call(
+            self,
+            argv: List[str], cwd: Optional[str] = None,
+            user: Optional[str] = None,
+            env: Optional[Dict[str, str]] = None):
+        raise NotImplementedError(self.check_call)
+
+    def check_output(
+            self,
+            argv: List[str], cwd: Optional[str] = None,
+            user: Optional[str] = None,
+            env: Optional[Dict[str, str]] = None) -> bytes:
+        raise NotImplementedError(self.check_output)
+
+    def Popen(self, argv, cwd: Optional[str] = None,
+              user: Optional[str] = None, **kwargs):
+        raise NotImplementedError(self.Popen)
+
+    def call(
+            self, argv: List[str], cwd: Optional[str] = None,
+            user: Optional[str] = None):
+        raise NotImplementedError(self.call)
+
+    def create_home(self) -> None:
+        """Create the user's home directory."""
+        raise NotImplementedError(self.create_home)
+
+
+class SessionSetupFailure(Exception):
+    """Session failed to be set up."""