Nowadays, many organizations form dynamic partnerships to deal effectively with market requirements. As companies use automated workflow management systems (WFMSs) to control their processes, a way of linking workflow processes in different organizations is required for turning the cooperating companies into a seamless operating virtual enterprise. The CrossFlow Esprit project aims at developing information technology for advanced workflow support in dynamic virtual organizations with contract based service trading. Contracts are necessary for flexible service outsourcing. This paper presents contracts as the basis for finding suitable partners, connecting WFMSs of different kinds, controlling outsourced workflow, and sharing an abstraction of the workflow specification between partners. CrossFlow contracts define the data, process, and conditions relevant to the cooperation and the outsourced workflow process on an abstract level. This information can be fed through an interface to the WFMSs of partners in a virtual enterprise to automate the co-operation between the partners completely.