Abstract
This paper introduces action refinement in the context of CSP||B. Our motivation to include this notion of refinement within the CSP parallel to B framework is the desire to increase flexibility in the refinement process. We introduce the ability to change the events of a CSP process and the B machines when refining a system. Notions of refinement based on traces and on traces/divergences are introduced in which abstract, events arc, refined by sequences of concrete events. A complementary notion of refinement between B machines is also introduced, yielding compositionality results for refinement of CSP parallel to B controlled components. The paper also introduces a notion of I/O refinement into our action refinement framework.