The vision of Model-Driven Engineering places models as first-class artifacts throughout the software lifecycle. An essential prerequisite is the availability of proper transformation languages allowing not only code generation but also augmentation, migration or translation of models themselves. Current approaches, however, lack convenient facilities for debugging and ensuring the understanding of the transformation process. To tackle these problems, we propose a novel formalism for the development of model transformations which is based on Colored Petri Nets. This allows first, for an explicit, process-oriented execution model of a transformation, thereby overcoming the impedance mismatch between the specification and execution of model transformations, being the prerequisite for convenient debugging. Second, by providing a homogenous representation of all artifacts involved in a transformation, including metamodels, models and the actual transformation logic itself, understandability of model transformations is enhanced.