State-of-the-art mobile computing environments must deal with scarce and dynamically varying resources-in particular, the network quality of service. Applications which execute in such environments need to adapt to the dynamic operating conditions in order to preserve the illusion of seamlessness for the end-user as far as possible. The paper proposes a framework for adaptation which provides applications with runtime support for quality of service negotiation, monitoring, and notification. Applications only need to specify the policy of adaptation at a high level, and are shielded from the mechanics of adaptation. The paper describes the design and implementation of the framework of adaptation as a part of the Prayer mobile computing environment.