Recent years have witnessed the development of a wide-range of wireless technologies. Next generation communication systems are aimed at integrating existing and emerging wireless systems to enable ubiquitous network access to mobile end-users. The availability of various access networks requires a mobile device to be capable of selecting the best access network with minimum user intervention. We present a design architecture called TAILOR that efficiently implements automatic network selection based on several factors such as the conditions of accessible networks, requirements of active applications, and preferences of the mobile user. We also present a case study to demonstrate the operation of the prototype TAILOR architecture in a real heterogeneous network testbed consisting of IEEE 802.11b and general packet radio service (GPRS) networks.