The ECOOP '91 Workshop on Object-Based Concurrent Computing was organized toprovide a forum on concurrent, distributed and open-ended computing. The emphasis was on conceptual, theoretical and formal aspects, as well as practical aspects and sound experience, since such a viewpoint was deemed indispensible to investigate and establish a basis for future development. This volume contains 12 papers selected from 25 presented at the workshop, together with a paper by J.A. Goguen, who was an invited speaker at the workshop. The papers are classified into four categories: Formal methods (1): three papers are concerned with the formal semantics of concurrent objects based on process calculi. Formal methods (2): four papers are concerned with various formal approaches to the semantics of concurrent programs. Concurrent programming: three papers. Models: three papers areconcerned with models for concurrent systems.