Mobile devices are used in a variety of different situations, for example at work, at home, or while traveling. However, specific security requirements of the current context are usually not considered. Even if security mechanisms apply for a certain situation, their activation usually remains a manual task. Our work investigates the use of context information to automatically and transparently adapt security facilities, aiming at increasing security (with a focus on usage control) of mobile devices. To this end, we propose a context-model for specification and a context-awareness framework for evaluation of context information. We use the context evaluation engine to enhance the decision making of our existing policy enforcement framework.