Designing understandable business process models is one of the key factors to successful business process management. Current modeling practices advocate the use of block-oriented concepts and sub-processes to structure complex process models. However, such guidelines cannot be applied in any case as case studies in process mining have shown. Previously, the authors proposed the scenario based paradigm to structure models of complex processes in behavioral fragments, i.e., scenarios. This paper presents GRETA as a tool that supports scenario-based process modeling and execution.