Access control and multi-level security are the bread and butter of computer security, and more generally, resource security. A resource is a one-way function: something that is easy to use but hard to come by. A resource can be secured if access to it can be controlled. If it cannot be secured, it cannot be owned. It is all simple and obvious at first sight. But there may be many different ways to access a resource, and many ways to own some of it. This chapter provides a quick tour of the tools used to manage such subtleties.