The ability to reuse transformations across a range of related metamodels is highly desired for many model-driven approaches. For example, it would be useful to be able to reuse standard transformations like state-machine attening across models instantiating dierent metamodels of hierarchical state-machines, instead of having to reimplement the same fundamental algorithm just because of small syntactic (or semantic) variations. Typing typically captures the question of identifying models for which a given transformation can be successfully applied. Type compatibility between meta-models (or the related notion of sub-typing), is intended to ensure that a transformation dened over one type can be successfully executed over any model instantiating a compatible meta-model. However, compatibility mechanisms have not explicitly addressed the question of what it makes for a transformation to be successfully applied. In this paper, we argue that answering this question is central to a me...