[Superseded by NISTIR 7987 Revision 1 (October 2015): http://www.nist.gov/manuscript-publication-search.cfm?pub_id=913195
] The ability to control access to sensitive data in accordance with policy is perhaps the most fundamental security requirement. Despite over four decades of security research, the limited ability for existing access control mechanisms to enforce a comprehensive range of policy persists. While researchers, practitioners and policy makers have specified a large variety of access control policies to address real-world security issues, only a relatively small subset of these policies can be enforced through off-the-shelf technology, and even a smaller subset can be enforced by any one mechanism. This report describes an access control framework, referred to as the Policy Machine (PM), which fundamentally changes the way policy is expressed and enforced. The report gives an overview of the PM and the range of policies that can be specified and enacted. The report also describes the architecture of the PM and the properties of the PM model in detail.