The research objective of this work is the theoretical founding of formalisms of knowledge representation, belief revision and defeasible reasoning, with focus in contextual reasoning. These formalisms will be developed in order to be used in complex, dynamic and evolving application scenarios, such as the scenarios appearing in the Semantic Web and Ambient Intelligence applications. Our formalisms will be evaluated both theoretically, via determining their theoretical properties, and practically, via experiments evaluating their effectiveness and intuitiveness. Applicability, efficiency and integration of multiple heterogeneous components constitute critical characteristics, in which both partners will focus.