This book constitutes the refereed proceedings of the 25th International Conference on Logic Programming, ICLP 2009, held in Pasadena, CA, USA, in July2009. The 29 revised full papers together with 9 short papers, 4 invited talks, 4 invited tutorials, and the abstracts of 18 doctoral consortium articles were carefully reviewed and selected from 69 initial submissions. The papers cover all issues of current research in logic programming, namely semantic foundations, formalisms, nonmonotonic reasoning, knowledge representation, compilation, memory management, virtual machines, parallelism, program analysis, program transformation, validation and verification, debugging, profiling, concurrency, objects, coordination, mobility, higher order, types, modes, programming techniques, abductive logic programming, answer set programming, constraint logic programming, inductive logic programming, alternative inference engines and mechanisms, deductive databases, data integration, software engineering, natural language, web tools, internet agents, artificial intelligence, bioinformatics.