Refinement is one of the cornerstones of a formal approach to software engineering. Refinement is all about turning an abstract description (of a soft or hardware system) into something closer to implementation. It provides that essential bridge between higher level requirements and an implementation of those requirements. This book provides a comprehensive introduction to refinement for the researcher or graduate student. It introduces refinement in different semantic models, and shows how refinement is defined and used within some of the major formal methods and languages in use today. It (1) introduces the reader to different ways of looking at refinement, relating refinement to observations(2) shows how these are realised in different semantic models (3) shows how different formal methods use different models of refinement, and (4) how these models of refinement are related. .