This book constitutes the refereed proceedings of the 9th International Conference on Logic Programming and Nonmonotonic Reasoning, LPNMR 2007, held in Tempe, AZ, USA in May 2007.The 18 revised full papers, 5 revised poster papers, and 7 system descriptions presented together with 2 invited papers were carefully reviewed and selected for presentation. As a forum for exchanging ideas on declarative logic programming, nonmonotonic reasoning, and knowledge representation, LPNMR encompasses theoretical studies, design and implementation of logic based programming languages and database systems, and development of experimental systems. Also contained in this volume are the results of this year's answer set programming competition.