Model-Driven Engineering (MDE) is becoming a widely accepted paradigm for the design and development of complex distributed applications. MDE advocates the use of models and model transformations as key artefacts in all phases of the software process, from system specification and analysis, to design, development and testing. Each model usually addresses one concern, independently from the rest of the issues involved in the construction of the system. Thus, the basic functionality of the system can be separated from its final implementation, and the business logic can be separated from the underlying platform technology, etc. Transformations between models enable the automated implementation of a system from the different models defined for it.