Current mobile devices enable the use of information systems while the user is on the move. Hence, it is necessary to deal with space and time continuously. Yet, mobile applications need to be aware and adapt themselves to highly dynamic environments. Contextaware applications use mobile computing in order to provide information to users according to their context anywhere at anytime. Modeling context implies modeling user behavior, movements and necessities. Ontologies seem to be a good candidate for modeling context-aware applications, as rich semantic relationships can be implemented. In this paper, we present an ontology-based approach to deal with context-aware systems.