This volume constitutes the thoroughly refereed post-conference proceedings of the Twelfth Advances in Computer Games Conference, ACG 2009, held in Pamplona, Spain, in May 2009. The 20 revised full papers presented were carefully reviewed and selected from 41 submissions for inclusion in the book. The topics addressed contain Monte-Carlo tree search, Bayesian modeling, selective search, brute force, conflict resolution, solving games, optimization, concept discovery, incongruity theory, and data assurance.