CONCUR'91 is the second international conference on concurrency theory, organised in association with the NFI project Transfer. It is a sequel of the CONCUR'90 conference. Its basic aim is to communicate ongoing work in concurrency theory. This year 71 papers were submitted, 17 more than last year. From these, 30 papers were selected for presentation at the conference. The selected papers appear in these proceedings, together with four papers of invited speakers. Furthermore, abstracts of the other invited lectures are included.