In this paper we present a calculus for re nement of business process models based on a precisede nition of business processes and process nets Business process models are a vital concept for communicating with experts of the application domain Depending on the roles and responsibilities of the application domain experts involved process models are discussed on different levels of abstraction These may range from detailed regulations for process execution to the interrelation of basic core processes on a strategic level To ensure consistency and to allow for a exible integration of process information on di erent levels of abstraction we introduce re nement rules that allow the incremental addition to and re nement of the information in a process model while maintaining the validity of more abstract high level processes In particular we allow the decomposition of single processes and logical data channels as well as the extension of the interface and channel structure to information that is newly gained or increased in relevance during the modeling process.