The paper shows how to refine large-scale or even infinite transition systems so as to ensure certain desired properties. First, a given system is reduced into a smallish, finite bisimulation quotient. Second, the reduced system is refined in order to ensure a given property, using any known finite-state method. Third, the refined reduced system is expanded back into an adequate refinement of the system given initially. The proposed method is based on a Galois connection between systems and their quotients. It is applicable to various models and bisimulations and is illustrated with a few qualitative and quantitative properties.