This paper presents an overview of the ACTS project RAINBOW which demonstrates a generic UMTS network architecture which is able to handle different radio access techniques. The approach pursued within the project is to develop a generic interface between the UMTS core network and the radio access parts. The basic concepts of the project are described, as well as the demonstrator built to evaluate these concepts and the services supported. Results from conceptual studies are also provided, which have been undertaken to further explore the project concepts and to evaluate alternative approaches.