In this paper, we describe three key problems for trust management in federated systems and present a layered architecture for addressing them. The three problems we address include how to express and verify trust in a flexible and scalable manner, how to monitor the use of trust relationships over time, and how to manage and reevaluate trust relationships based on historical traces of past behavior. While previous work provides the basis for expressing and verifying trust, it does not address the concurrent problems of how to continuously monitor and manage trust relationships over time. These problems close the loop on trust management and are especially relevant in the context of federated systems where remote resources can be acquired across multiple administrative domains and used in potentially undesirable ways (e.g., to launch denial-of-service attacks).