This volume contains the papers presented at CONCUR 2005, the 16th - ternational Conference on Concurrency Theory. The purpose of the CONCUR series of conferences is to bring together researchers,developers, and students in order to advance the theory of concurrency and to promote its applications. This year's conference was in San Francisco, California, from August 23 to August 26. We received 100 submissions in response to a call for papers. Each subm- sionwasassignedto at leastthreemembers ofthe ProgramCommittee; in many cases, reviews were solicited from outside experts. The ProgramCommittee d- cussed the submissions electronically, judging them on their perceived imp- tance, originality, clarity, and appropriateness to the expected audience. The Program Committee selected 38 papers for presentation. Because of the format of the conference and the high number of submissions, many good papers could not be included. Although submissions werereadand evaluated, the papers that appear in this volume may di?er in form and contents from the corresponding submissions. It is expected that many of the papers will be further revised and submitted to refereed archival journals for publication.