This book constitutes the refereed proceedings of the 12th International Conference on Principles and Practice of Constraint Programming, CP 2006, held in Nantes, France in September 2006.

The 42 revised full papers and 21 revised short papers presented together with extended abstracts of 4 invited talks were carefully reviewed and selected from 142 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.