Trust and authority are essential components that must be factored in designing and implementing a distributed access control mechanism for open environments. This paper presents a role-based approach to modeling trust and authority for open and distributed computing environments. Specifically, our approach centers on the extension of role-based capability delegation from local domains to trusted domains. A formal specification based on the fixpoint semantics is presented in order to better understand and design a distributed access control mechanism that can be built on our approach. Keyword: Distributed access control, role-based authorization, trust management.