The search for an appropriate characterization of negation as failurein logic programs in the mid 1980s led to several proposals. Amongstthem the stable model semantics – later referred to asanswer set semantics, and the well-foundedsemantics are the most popular and widely referred ones. Accordingto the latest (September 2002) list of most cited source documentsin the CiteSeer database (http://citeseer.nj.nec.com) theoriginal stable model semantics paper (Gelfond and Lifschitz, 1988)is ranked 10th with 649 citations and the well-founded semanticspaper (Van Gelder et al., 1991) is ranked 70th with306 citations. Since 1988 – when stable models semantics wasproposed – there has been a large body of work centered around logicprograms with answer set semantics covering topics such as:systematic program development, systematic program analysis,knowledge representation, declarative problem solving, answer setcomputing algorithms, complexity and expressiveness, answer setcomputing systems, relation with other non-monotonic and knowledgerepresentation formalisms, and applications to various tasks.