This volume contains the proceedings of the 20th Conference on Concurrency Theory (CONCUR 2009), held in Bologna, September 1-4, 2009. The purpose of the CONCUR conference is to bring together researchers, developers, and s- dentsinordertoadvancethetheoryofconcurrencyandpromoteitsapplications. This year the CONCUR conference was in its 20th edition, and to celebrate 20 years of CONCUR, the conference program included a special session organized by the IFIP Working Groups 1.8 "Concurrency Theory" and 2.2 "Formal - scriptionofProgrammingConcepts"aswellas aninvitedlecturegivenby Robin Milner, one of the fathers of the concurrency theory research area. This edition of the conference attracted 129 submissions. We wish to thank all their authors for their interest in CONCUR 2009. After careful discussions, the Program Committee selected 37 papers for presentation at the conference. Each of them was accurately refereed by at least three reviewers (four reviewers for papers co-authored by members of the Program Committee), who delivered detailedandinsightfulcommentsandsuggestions.TheconferenceChairswarmly thank all the members of the Program Committee and all their sub-referees for the excellent support they gave, as well as for the friendly and constructive discussions. We would also like to thank the authors for having revised their papers to address the comments and suggestions by the referees. The conference program was enriched by the outstanding invited talks by Martin Abadi, Christel Baier, Corrado Priami and, as mentioned above, Robin Milner.