The ECOOP '91 Workshop on Object-Based Concurrent Computingwas organized toprovide a forum on concurrent, distributedand open-ended computing. The emphasis was on conceptual,theoretical and formal aspects, as well as practical aspectsand sound experience, since such a viewpoint was deemedindispensible to investigate and establish a basis forfuture development. This volume contains 12 papers selected from 25 presented atthe workshop, together with a paper by J.A. Goguen, who wasan invited speaker at the workshop. The papers areclassified into four categories:Formal methods (1): three papers are concerned with theformal semantics of concurrent objects based on processcalculi. Formal methods (2): four papers are concerned with variousformal approaches to the semantics of concurrent programs. Concurrent programming: three papers. Models: three papers areconcerned with models forconcurrent systems.