Software metrics measure the quality of code according to criteria like reusability, understandability and well-structuredness. Refactorings change code as to improve the quality while preserving overall behaviour. Measuring and refactoring go hand in hand as particular results of measurements indicate specific design problems which suggest certain refactorings. As a consequence, we get a quality cycle of repeated steps of measuring, diagnosing and refactoring.In this paper, we transfer such a quality cycle to the area of software models. Contrary to code, models - especially UML models - usually do not consist of a single entity but out of several diagrams. Our measures and refactorings jointly treat these diagrams. Contrary to code refactorings, we furthermore base the refactorings on a formal semantics for models and only employ refactorings which provably preserve the behaviour. The approach has been implemented in the tool RMC, supporting the definition as well as execution of quality measurements and refactorings.