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. This book includes the 18 revised full papers, 5 revised poster papers, and 7 system descriptions together with 2 invited papers that are reviewed and selected for presentation.

Invited Talks/Competition.- Logic Programming and Nonmonotonic Reasoning: From Theory to Systems and Applications.- Policy-Based Computing: From Systems and Applications to Theory.- The First Answer Set Programming System Competition.- Technical Papers.- cr-models: An Inference Engine for CR-Prolog.- Debugging ASP Programs by Means of ASP.- A Purely Model-Theoretic Semantics for Disjunctive Logic Programs with Negation.- Complexity of Default Logic on Generalized Conjunctive Queries.- A Preference-Based Framework for Updating Logic Programs.- Well-Founded Semantics and the Algebraic Theory of Non-monotone Inductive Definitions.- On the Complexity of Answer Set Programming with Aggregates.- Experimenting with Look-Back Heuristics for Hard ASP Programs.- Complexity of Rule Redundancy in Non-ground Answer-Set Programming over Finite Domains.- Conflict-Driven Answer Set Enumeration.- Head-Elementary-Set-Free Logic Programs.- A Deductive System for PC(ID).- Modularity Aspects of Disjunctive Stable Models.- A Characterization of Strong Equivalence for Logic Programs with Variables.- A Unified Semantics for Constraint Handling Rules in Transaction Logic.- Conditional Planning with External Functions.- Logic Programs with Abstract Constraints: Representaton, Disjunction and Complexities.- General Default Logic.- System Descriptions.- The System: Logic Programming Meets Outlier Detection.- clasp: A Conflict-Driven Answer Set Solver.- GrinGo: A New Grounder for Answer Set Programming.- Using Answer Sets to Solve Belief Change Problems.- An Smodels System with Limited Lookahead Computation.- Programming Applications in CIFF.- CPP: A Constraint Logic Programming Based Planner with Preferences.- Posters.- An Application of Defeasible Logic Programming to Decision Making in a Robotic Environment.- On the Effectiveness of Looking Ahead in Search for Answer Sets.- Enhancing ASP Systems for Planning with Temporal Constraints.- Semantics for Possibilistic Disjunctive Programs.- Modularity in smodels Programs.