Distributed Role-Based Access Control (dRBAC) is a scalable, decentralized trust-management and access-control mechanism for systems that span multiple administrative domains. dRBAC represents controlled actions in terms of roles, which are defined within the trust domain of one entity and can be transitively delegated to other roles within a different trust domain. dRBAC utilizes PKI to identify all entities engaged in trust-sensitive operations and to validate delegation certificates. The mapping of roles to authorized name spaces obviates the need to identify additional policy roots. dRBAC distinguishes itself from previous trust management and role-based access control approaches in its support for three features: (1) third-party delegations, which improve expressiveness by allowing an entity to delegate roles outside its namespace when authorized by an explicit delegation of assignment; (2) valued attributes, which modulate transferred access rights via mechanisms that assign and manipulate numerical values associated with roles; and (3) credential subscriptions, which enable continuous monitoring of established trust relationships using a pub/sub infrastructure to track the status of revocable credentials. This paper describes the dRBAC model, its scalable implementation using a graph-based model of credential discovery and validation, and its application in a larger security context.