Conventional object-oriented data models are closed: although they allow users to define application-specific classes, they usually come with a fixed set of modelling primitives. This constitutes a major problem, as different application domains, e.g. database integration or multimedia, need special support. Using an extended metaclass concept, this book provides for the solution of this problem a simple but extendible open object-oriented data model, a so-called RISC model. By introducing the basic concepts of the open object-oriented database management system VODAK, it demonstrates how the extended metaclass concept can be integrated homogeneously into object-oriented data models.