This book constitutes the refereed conference proceedings of the 18th International Conference on Principles and Practice of Constraint Programming (CP 2013), held in Uppsala, Sweden, in September 2013. The 61 revised papers presented together with 3 invited talks were carefully selected from 138 submissions. The scope of the conference is on all aspects of computing with constraints, including: theory, algorithms, environments, languages, models and systems, applications such as decision making, resource allocation, and agreement technologies.