There has been a large and varied amount of research into federated database systems, and more recently, this has advanced in the direction of federations of multimedia databases. However, in each project studied so far, none have placed special emphasis on the role of metadata in such systems. In this paper, a detailed specification of a multimedia metamodel suitable for a federation of databases, is provided. By illustrating the metamodel, it is clear to users, administrators and programmers exactly how data is structured, and the types of operations that are possible.