The next generation of mobile wireless system is expected to allow a multimode terminal to simultaneously access multiple networks for a variety of services with an appropriate quality of service. To achieve such a goal, many technologies such as multimode terminals, QoS-based network selection, and mobility management should be effectively incorporated into wireless network systems. This article discusses the current progress in network architecture as well as the functionalities of a multimode terminal, and identifies the fundamental research issues for future work in such a heterogeneous wireless environment.