Nonmonotonic logics were created as an abstraction of some types of common sense reasoning, analogous to the way classical logic serves to formalize ideal reasoning about mathematical objects. These logics are nonmonotonic in the sense that enlarging the set of axioms does not necessarily imply an enlargement of the set of formulas deducible from these axioms. Such situations arise naturally, for example, in the use of information of different degrees of reliability. This book emphasizes basic concepts by outlining connections between different formalisms of nonmonotonic logic, and gives a coherent presentation of recent research results and reasoning techniques. It provides a self-contained state-of-the-art survey of the