Workflow management systems support the execution of business processes. To this end they need data, data about the business cases, data for performing steps of the process, data for making decisions in the control of the process. Workflows generate and consume data, particularly data stored in external databases and used by different applications. This book makes three important contributions to data handling in workflow management systems. First, it proposes a uniform treatment of all kinds of business data in workflows and describes an abstraction layer which enables the transparent access to data in any source in a uniform way. Then it extends the functionality of workflow management systems with a manageable and policy based mechanism for maintaining copies of external data in the workflow repository. Finally, it presents a method of ensuring data correctness in workflows. All these concepts are incorporated into a prototype workflow definition language and presented in a way that makes them applicable to other languages and workflow management systems as well.