Circus is a combination of Z, CSP, and the refinement calculus, and is based on Hoare & He’s Unifying Theories of Programming. A model checker is being constructed for the language to conduct refinement checking in the style of FDR, but supported by theorem proving for reasoning about the complex states and data types that arise from the use of Z. FDR deals with bounded labelled transition systems (LTSs), but the Circus model checker manipulates LTSs with possibly infinite inscriptions on arcs and in nodes, and so, in general, the success or failure of a refinement check depends on interaction with a theorem prover. An LTS is generated from a source text using an operational interpretation of Circus; we present a Structured Operational Semantics for Circus, including both its process-algebraic and state-rich features.