This volume contains the 33 papers presented at IPCO 2011, the 15th Conference on Integer Programming and Combinatorial Optimization, held during June 15–17, 2011 at the IBM T.J. Watson Research Center in New York, USA. IPCO conferences are sponsored by the Mathematical Optimization Society. The first IPCO conference took place at the University of Waterloo in May 1990. A lot of things have changed over the course of the 20 years since then. IPCO has become extremely visible, and it has grown into one of the main events for the mathematical programming community. IPCO has also become bigger in size (for instance, the number of submissions has increased from 76 to 110, and the number of Program Committee members has increased from 8 to 14). And IPCO has become considerably broader in scope (nowadays the meeting covers topics that did not even exist in 1990). Of course there are other things that remained constant over time. For instance, the format of IPCO is still a single stream of non-parallel sessions (this has become IPCO tradition, and is unlikely to change in the near future). The number of accepted papers still is 33 (plus/minus 3). And – most important – the quality of the presented research is at the very very top. Altogether IPCO 2011 received 110 extended abstracts. Each submission was reviewed by at least three, and on average by four, Program Committee members prior to the committee meeting in January 2011 in Aussois. During the committee meetings all 110 submission were discussed thoroughly and 33 papers were selected for presentation. We expect the full versions of the papers contained in this volume to be submitted for publication in refereed journals.