This volume contains the papers presented at the 6th Refinement Workshop which was held in London from 5 - 7 January 1994. The theme of the workshop was "the theory and practice of formal software development", and papers with industrial relevance were particularly encouraged. The papers address both the formal mathematical infrastructure of methods, and the practical application of formal development methods in commerce and industry. Among the specific topics covered are the formal development of authentication protocols, operation semantics with read and write frames, and the testing and safety analysis of abstract machine specifications. This volume will be of interest to academic and industrial researchers, postgraduate students, and research-oriented developers in the computer industry. The proceedings of the previous three Refinement Workshops are also available in the series.