This book constitutes the proceedings of the 12th International Conference on Principles of Practice in Multi-Agent Systems, PRIMA 2009, held in Nagoya, Japan, in December 2009. The 34 papers presented together with 18 poster papers were carefully reviewed and selected from over 100 submissions. They focus on practical aspects of multiagent systems and cover topics such as participatory simulation, practical auctions, ontology building and framework evaluation.