The quality of software systems depends on many criteria. Choosing an adequate software process model is essential in meeting quality requirements. When aiming at a rigorous mathematical notion of correctness, formal software development provides an appropriate process model, which ensures that the implementation fulfills the requirements given by a formal specification. The stepwise derivation by transformations represents one approach to formal system development that can be supported by computers. This book first introduces a sound framework for the mechanizable transformation of formal specifications. The framework comprises a specification language, a formal development method and various mechanizable transformation rules. It is then illustrated how the framework is implemented in the Lubeck Transformation System LTS -- a tool for the interactive development of software systems.