Recent efforts to federate computation and communication resources across organizational boundaries face a challenge in establishing the policies by which one organization's users can access resources in other organizations. This paper describes an approach to defining, communicating, analyzing, and enforcing resource allocation policies in this new setting. Our approach was designed to address the needs of PlanetLab, but we demonstrate through a range of examples that it is general enough to accommodate a diverse collection of computing facilities. Our policy engine is implemented in a specific tool chain, called sfatables, that is patterned after the iptables mechanism used to define packet processing policies for network traffic. The interface to our policy engine thus uses the familiar paradigm of a firewall and provides a flexible interface for resource owners to specify access policies for their resources. Our implementation makes it possible to precisely document policies, query, and analyze them.