This book constitutes the refereed proceedings of the 15th International Conference on Principles and Practice of Constraint Programming, CP 2009, held in Lisbon, Portugal in September 2009. The 61 revised full papers presented together with the abstracts of three invited talks were carefully reviewed and selected from 141 submissions. All current issues of computing with constraints are addressed, ranging from methodological and foundational aspects - using algorithms, environments, languages, models and systems - to solving real-world problems in various application fields.