Mobile devices, such as mobile phones and PDAs, have gained wide-spread popularity. Applications for this kind of mobile devices have to adapt to changes in context, such as variations in network bandwidth, battery power, connectivity, reachability of services and hosts, and so on. In this paper, we define context-aware action systems that provides a systematic method for managing and processing context information. The meaning of context-aware action systems is defined in terms of classical action systems, so that the properties of context-aware action systems can be proved using standard action systems proof techniques. We describe the essential notions of this formalism and illustrate the framework with examples on context-aware services for mobile applications.