  This entry assumes an audience consisting primarily of philosophers who have little or no familiarity with AI. The entry concentrates on the issues that arise when logic is used in understanding problems in intelligent reasoning and guiding the design of mechanized reasoning systems. Logic in AI is by now a very large and not very well demarcated field—nothing like complete coverage has been achieved here.   Sections 3  and  Section 4  provide an overview with some historical and technical details concerning nonmonotonic logic and reasoning about action and change, a topic that is not only central in AI but that should be of considerable interest to philosophers. The remaining sections provide brief and more or less inadequate sketches of selected topics, with references to the primary literature.