Workflow management systems (WfMS) are designed to enable the enactment of business processes, i.e. to extract and maintain the process logic between business applications. Within a workflow management solution, heterogeneous applications are connected through a given workflow definition. Future development efforts must aim to establish interoperability between different WfMS. This leads straight to interoperability problems of two types: in a technical point of view, one WfMS has to be able to abstract from a concrete implementation (programming language, hardware, etc.) of another WfMS. In a semantic point of view, WfMS have to agree on the execution semantics of a workflow definition. We try to cope with the semantic interoperability problem by defining a workflow repository which enables extensible communication between WfMS. To understand the notion and the central function of repositories, a short history of metadata systems is given. Based on a modular architecture of WfMS, a repository is developed and sketched in a CORBA compliant style. This repository shows the expressiveness and the idea of extensibility of our interoperability approach.