This landmark volume represents the culmination of over 40 years of research in the use of logic as a basis for representing and manipulating problems in the field of artificial intelligence. The use of logic as a basis for commonsense reasoning was started by John McCarthy in 1959. The collection consists of both original research and surveys of almost every subject that uses logic in AI, contributed by leading scientists, and grew out of preliminary work presented at the Workshop on Logic-Based Artificial Intelligence held in Washington, DC, June 1999. All papers have been extensively refereed and revised.