This volume contains the proceedings of the 21st Conference on C- currency Theory (CONCUR 2010), held in Paris, at UniversiteDenis Diderot, August 31 September 3, 2010. CONCUR 2010 was organized by CNRS, INRIA, ENS Cachan and the UniversiteDenisDiderot. The purpose of the CONCUR conference is to bring together researchers, developers, and students in order to advance the theory of concurrency and promote its applications. The principal topics include basic models of concurrency such as abstract machines, domain theoretic models, game theoretic models, process algebras, and Petri nets; logics for concurrency such as modal logics, probabilistic and stochastic logics, temporal logics, and resource logics; models of specialized systems such as biology-inspired systems, circuits, hybrid systems, mobile and collabo- tive systems, multi-core processors, probabilistic systems, real-time s- tems, service-oriented computing, and synchronous systems; veri?cation and analysis techniques for concurrent systems such as abstract int- pretation, atomicity checking, model checking, race detection, pre-order and equivalence checking, run-time veri?cation, state space exploration, static analysis, synthesis, testing, theorem proving, and type systems; - lated programming models such as distributed, component-based, obje- oriented, and Web services. This edition of the conference attracted 107 submissions. We wish to thank all their authors for their interest in CONCUR 2010. After careful discussions, the Program Committee selected 35 papers for presentation at the conference. Each submission was refereed by three reviewers, who delivered detailed and insightful comments and suggestions."