We propose a formal model for reputation-based trust management. In contrast to conventional, credential-based trust management, in reputation-based trust management an agent’s past behavior is used to decide whether to grant him access to a protected resource. We use an event semantics inspired by the actor model, and assume that each principal has only partial knowledge of the events that have occurred. Licenses are used to formalize restrictions on agents’ behavior. If an agent fulfills the obligations imposed by a license and does not misuse the license by performing a forbidden action, then his reputation improves. This approach enables precise formal modeling of scenarios involving reputations, such as financial transactions based on credit histories and information sharing between untrusted agents.