Workflow management systems support the execution of business processes: they require the modelling of the processes, they drive the actual enactment of process instances, and they document the business process execution. Many different modelling languages and systems have been developed for the design and representation of business processes and workflows as a consequence of different requirements, different purposes, as well as different schools and modelling philosophies. The differences are partly on a conceptual level in the sense that the modelling languages offer different concepts and constructs, and partly on the representation level where basically the same constructs are represented differently (e.g. text- or graph-based). This book makes two important contributions to workflow modelling. Firstly, it provides a metamodel based on abstract modelling concepts, which copes with representation differences of workflow descriptions. Secondly, it introduces a set of equivalence transformations on workflow models. Such transformations are an important tool for workflow developments as they allow representing workflows differently for different purposes. These transformations are used in algorithms for checking the satisfiability of temporal constraints in workflow definitions. Using this apparatus made it possible to develop a sophisticated time management system for workflows, which not only checks temporal constraints but can also be used to develop time plans for workflow execution.