The main focus of this project is on the development of a formal automated mechanism for the implementation of default reasoning in expert systems. The formal structures are used to provide a deeper understanding of the default reasoning process and of nonmonotonicity. This formal structure is developed in the framework of the fuzzy set based theory of approximate reasoning. Goals of this project include the representation of various types of default rules by possibility qualified statements, the implementation of the operation possibility qualification, the effect of multiple defaults on the reasoning process, a comparison of probability and default reasoning systems, and the modeling conjunction of fuzzy and default environments.