The papers contained in this volume were presented at the 3rd Refinement Workshop which was held at IBMs Hursley Park Laboratory, from 9-11 January 1990. In January 1988, Professor John McDermid, of the University of York, organised a two-day workshop on the Theory and Practice of Refinement. The purpose of this first workshop was to focus attention on refinement and expedite work in the area. Since then, the 2nd and 3rd Refinement Workshops have been held and the 4th Refinement Workshop is planned for January 1991. Refinement, in its most general sense, is the extraction of programs from specifications. The objective of work in the refinement area is the systematic and verified development of implementations from specifications, be they hardware or software developments.