The purpose of the book is to give a unified and comprehensive account of default logic, the most popular logic for those in the Artificial Intelligence (AI) community interested in the formalization of reasoning with incomplete information. The book is mainly concerned with a systematic presentation of the formal theory of default logic, even though the more informal issue of applications of default logic to Artificial Intelligence in general and Knowledge Representation in particular is extensively dealt with, especially by means of many illustrative examples. The book also contains an overview of the other main logics for reasoning in the absence of complete information about the world. The book is intended to be self-contained, so that it is suitable for beginners. As a textbook it is mainly aimed at graduate students for a course on nonmonotonic reasoning. It is also meant to serve as a reference book for AI workers and for researchers in various fields, e.g. Artificial Intelligence, philosophy and cognitive psychology.