In object-oriented programming, continuous refactorings are used as the main mechanism to increase the maintainability of the code base. Unfortunately, in the field of model transformations, such refactoring support is so far missing. This paper tackles this limitation by adapting the notion of refactorings to model-to-model (M2M) transformations. In particular, we present a dedicated catalogue of refactorings for improving the quality of M2M transformations. The refactorings have been explored by analyzing existing transformation examples defined in ATL. However, the refactorings are not specifically tailored to ATL, but applicable also to other M2M transformation languages.