An access control model describes at a high level of abstraction a mechanism for governing access to shared resources. In this paper, we view an access control model as a design pattern providing a general solution for ensuring confidentiality, integrity and availability of information resources. We present three widely used access control models, DAC, MAC and RBAC as design patterns using the POSA template. We use an extension of the UML to represent the structure and behavior of the patterns. The extension enables capturing variations of pattern instances. We also attempt to give more details on the problem domain of the patterns to help pattern selection.