The book "Concur 2009 - Concurrency Theory" documents the proceedings of the 20th Conference on Concurrency Theory, which took place from September 1 to 4, 2009, in Bologna. This conference brings together researchers, developers, and students to advance the theory of concurrency and promote its applications. In celebration of the 20th anniversary of the conference, the program included a special session organized by the IFIP Working Groups 1.8 and 2.2, as well as an invited talk by Robin Milner, one of the pioneers in concurrency research. The conference attracted 129 submissions, of which 37 were selected for presentation after careful review. The contributions were evaluated by at least three reviewers, resulting in a high quality of the published works.