This volume contains the papers presented at CP 2009: The 15th International Conference on Principles and Practice of Constraint Programming. It was held from September 20-24, 2009 at the Rectory of the New University of Lisbon, Portugal. Everyone involved with the conference thanks our sponsors for their support. There were 128 submissions to the research track, of which 53 were accepted for a rate of 41.4%. Each submission was reviewed by three reviewers, with a small number of additional reviews obtained in exceptional cases. Each review waseitherbyaProgrammeCommitteemember,...