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,orbyacolleagueinvitedtohelp by a committee member thanks to their particular expertise. Papers submitted as long papers were accepted at full length or not at all. It is important to note that papers submitted as short papers were held to the same high standards of qualityas long papers. There is thus no distinction in these proceedings between long and short papers, except of course the number of pages they occupy. As it happens, the acceptancerates of short and long papers wereverysimilar indeed. Therewere13submissionstotheapplicationtrack,ofwhich8wereaccepted, fora rateof61.5%.Papersunderwentthe samereviewprocessasregularpapers, and there was not a separate committee for reviewing application track papers. However, papers in the application track were not required to be original or novel research, but to be original and novel as an application of constraints.