In software engineering the MDSD (Model Driven Software Development) approach has gained continuously increasing popularity in recent years. The basic idea of the MDSD technology is to drive the whole software development process by specialized models that correspond to each development phase. These models not only serve as documentation of various aspects of the system, but also become a direct constituent of the software. In practice, various specialized modeling languages are nowadays used for description of the MDSD models. These languages (called Domain Specific Languages or DSLs) allow describing models of some specific problem domain more clearly and effectively by using notation designed specifically for the given domain. Along with the introduction of MDSD in software development processes and its application to new problem domains, a constant need arises for creation of new DSLs and their editors. This book describes a universal tool building platform that allows simplified tool definition by creating a mapping between the metamodel of the graphical DSL and the metamodel of the tool being built, using model transformations for the definition of the correspondence.