Abstract
A theory of timewise refinement is presented. This allows the translation of specifications and proofs of correctness between semantic models, permitting each stage in the verification of a system to take place at the appropriate level of abstraction. The theory is presented within the context of CSP. A denotational characterisation is given in terms of relations between behaviours at different levels of abstraction, and various properties for the preservation of refinement through parallel composition are discussed. An operational characterisation is also given in terms of timed and untimed tests, and observed to coincide with the denotational characterisation.