In the area of model-driven engineering, model transformations are proposed as the technique to systematically manipulate models. For increasing development productivity as well as quality of model transformations, reuse mechanisms are indispensable. Although numerous mechanisms have been proposed, no systematic comparison exists, making it unclear, which reuse mechanisms may be best employed in a certain situation. Thus, this paper provides an in-depth comparison of reuse mechanisms in model-to-model transformation languages and categorizes them along their intended scope of application. Finally, current barriers and facilitators to model transformation reuse are discussed.