Context awareness has emerged as an important element in distributed computing. It offers mechanisms allowing applications to be aware of their environment and enabling them to adjust their behavior to the current context. In order to keep track of the relevant context information, a flexible service mechanism should be available for the client applications. In this paper we present a service architecture to provide contextawareness capabilities to users and client applications. Moreover, the service is able to react depending on the user?s preferences and context. The conditions for the reaction and the reaction itself are defined in rules the users submit to the service by means of a convenient rule language.