Model Driven Engineering (MDE) techniques support extensive use of models in order to manage the increasing complexity of software systems. Approp riate abstractions of software system elements can ease reasoning and understand ing and thus limit the risk of errors in large systems. Automatic model transfor mations play a critical role in MDE since they automate complex, tedious, error- prone, and recurrent software development tasks. A fault in a transformation can introduce a fault in the transformed model, which if undetected and not removed , can propagate to other models in successive development steps. As a fault prop agates further, it becomes more difficult to detect and isolate. Since model tra nsformations are meant to be reused, faults present in them may result in many f aulty models. Model transformations constitute a class of programs with unique c haracteristics that make testing them challenging. The complexity of input and o utput data, lack of model management tools, and the heterogeneity of transformat ion languages pose special problems to testers of transformations. In this paper we identify current model transformation characteristics that contribute to the difficulty of systematically testing transformations. We present promising solu tions and propose possible ways to overcome these barriers.