This monograph provides a thorough analysis of two importantformalisms for nonmonotonic reasoning: default logic andmodal nonmonotonic logics. It is also shown how they arerelated to each other and how they provide the formalfoundations for logic programming. The discussion isrigorous, and all main results are formally proved. Many ofthe results are deep and surprising, some of them previouslyunpublished. The book has three parts, on default logic, modalnonmonotonic logics, and connections and complexity issues,respectively. The study of general default logic is followedby a discussion of normal default logic and its connectionsto the closed world assumption, and also a presentation ofrelated aspects of logic programming. The general theory ofthe family of modal nonmonotonic logics introduced byMcDermott and Doyle is followed by studies of autoepistemiclogic, the logic of reflexive knowledge, and the logic ofpure necessitation, and also a short discussion ofalgorithms for computing knowledge and belief sets. Thethird part explores connections between default logic andmodal nonmonotonic logics and contains results on thecomplexity of nonmonotonic reasoning. The ideas are presented with an elegance and unity ofperspective that set a new standard of scholarship for booksin this area, and the work indicates that the field hasreached a very high level of maturity and sophistication. The book is intended as a reference on default logic,nonmonotonic logics, and related computational issues, andis addressed to researchers, programmers, and graduatestudents in the Artificial Intelligence community.