CONCUR'91 is the second international conference onconcurrency theory, organized in association with the NFIproject Transfer. It is a sequel to the CONCUR'90conference. Its basic aim is to communicate ongoing work inconcurrency theory. This proceedings volume contains 30papers selected for presentation at the conference (from 71submitted) together with four invited papers and abstractsof the other invited papers. The papers are organized intosections on process algebras, logics and model checking, applications and specification languages, models and nettheory, design and real-time, tools and probabilities, andprogramming languages. The proceedings of CONCUR'90 areavailable asVolume 458 of Lecture Notes in ComputerScience.