Mobile systems are gaining more and more importance, nevertheless the means for their specifications are still underdeveloped. Existing UML diagrams can be used to conveniently model behavior, but these diagrams can be hardly used to model mobility. In this paper we present an extension to UML class and activity diagrams to model mobile systems. We assume that mobile objects can migrate from one location to another. Locations can be nested and mobile too. We introduce stereotypes to model mobile objects, locations, and activities like moving or cloning. We introduce two notational variants of activity diagrams for modeling mobility. One variant is location centered and focuses on the topology of locations. The other one focuses on the actor responsible for an activity. We compare these two types of diagrams and define a metamodel for them.