In the study of CCS and related process calculi, there are two standard approaches to the definition of similarity or equivalence of processes. The first is bisimulation, an equivalence relation based on the structural operational semantics of the calculus. It has simulation as an asymmetric variant. The second is refinement, an ordering defined as inclusion of the sets of observations that may be made of the behaviour of each process. The original forms of bisimulation (strong and weak) were intended to give the strongest reasonable definition of process expressions.This paper shows how to define a process calculus in which the concepts of simulation and refinement coincide with each other : thus the combined benefits of two approaches are available uniformly to all applications.