This book constitutes the refereed proceedings of the 5th International Conference on Logic Programming and Nonmonotonic Reasoning, LPNMR '99, held in El Paso, Texas, USA, in December 1999. The volume presents 26 contributed papers and four invited talks, three appearing as extended abstracts and one as a full paper. Topics covered include logic programming, non-monotonic reasoning, knowledge representation, semantics, complexity, expressive power, and implementation and applicatons.