The search for an appropriate characterization of negation as failure in logic programs in the mid 1980s led to several proposals. Amongst them the stable model semantics – later referred to as answer set semantics, and the well-founded semantics are the most popular and widely referred ones. According to the latest (September 2002) list of most cited source documents in the CiteSeer database (http://citeseer.nj.nec.com) the original stable model semantics paper (Gelfond and Lifschitz, 1988) is ranked 10th with 649 citations and the well-founded semantics paper (Van Gelder et al., 1991) is ranked 70th with 306 citations. Since 1988 – when stable models semantics was proposed – there has been a large body of work centered around logic programs with answer set semantics covering topics such as: systematic program development, systematic program analysis, knowledge representation, declarative problem solving, answer set computing algorithms, complexity and expressiveness, answer set computing systems, relation with other non-monotonic and knowledge representation formalisms, and applications to various tasks.