Distribution and heterogeneity are the most relevant barriers for the development of interoperable information systems. By means of metadata-based middleware distributed and heterogeneous system components can be integrated, thus providing transparency while preserving autonomy. This paper presents a case study of three different projects in the area of metadata-based middleware and compares them along different criteria. Keywords: Distribution, Heterogeneity, Middleware, Transparency, Metadata. 1. INTRODUCTION Distribution and heterogeneity become increasingly important keywords characterizing today's information system infrastructure. They may be either a result of historical reasons, which means that certain system components have been developed independently, without having a later integration in mind or they may have their origin in organizational or technical circumstances. However, distribution and heterogeneity are the most relevant barriers for the development of interoperab..