During a software modernization project, a legacy system is transferred into and adapted to a new environment. A transformation method guides this endeavor by prescribing activities to perform or artifacts to generate in order to realize the transition. Thereby, the method used needs to fit to the situation at hand by considering conceptual differences between source and target environment and by automating parts of the transformation whenever suitable. Otherwise, a decreased software quality or increased effort to perform the transformation may result. Although various method engineering approaches have been proposed to support the development of transformation methods, most of them do not provide sufficient flexibility in the development or fall short in guiding the endeavor. In this thesis, we address this problem by introducing a situational method engineering framework to guide the development of situation-specific transformation methods. The framework uses a method base that contains reusable building blocks of transformation methods, based on principles from the domain of model-driven engineering. The development of a method is centered around the identification of concepts within a legacy system that represent its functionality by abstracting from the technology-specific realization. Selecting predefined, model-driven transformation strategies for each concept enables assembling transformation methods that are well-suited for a software modernization scenario.