Over the past two years, a series of research projects involving the formal use of contexts applied to information system modeling in general and computer security in particular has been supported by the Communications Security Establishment in Ottawa, Canada. The notion of context we have evolved is implemented in a language we call TENSAR (Taxonomies, ENtities, Sets and Relations). present, the context mechanism in TENSAR allows abstract models constructed from a general set theoretic foundation to have distinct context-dependent specifications. The TEWSAR interpreter stores all model information in a semantic frame knowledge-base that is partitioned by context. We have conjectured that an algebraic lattice of contexts would be appropriate for the applications envisaged, and the current implementation of TENSAR supports this.