The evolution of wireless communications during the past years, has led to the heterogeneous network environments, where various access technologies, such as WLAN, UMTS, DVB-T, Bluetooth, etc., are simultaneously available to mobile devices. Correspondingly, mobile terminals appear with more than one wireless interfaces, thus they become able to interact with any available network. This increase of the alternatives, in combination with the variety of applications, raises an issue, not present in traditional wireless systems, the access network selection problem. The lack of uniformity of the different network characteristics (coverage, service capabilities and compatibilities, security, power consumption, bandwidth, delay, etc) makes them more or less suitable for a specific application, depending also on the service needs and the current network conditions. This PhD thesis presents an integrated approach, network based and terminal assisted, for the access selection issue and generally for the mobility management of the users in the context of a composite radio environment.Initially, a suitable management system for heterogeneous networks is adopted, in order to physically place the proposed functionalities. The general architecture is quite simple and distributed. The main operations are assigned to the gateway or access router of each subnet. In parallel, all access networks' particularities are maintained. The technologies considered are UMTS, WLAN and DVB-T, with UMTS to be the home network for the users.Afterwards, the access selection problem is delimited by defining all the input and output data, the restrictions and the triggering policy. It is also mathematically formulated and an appropriate algorithm is chosen and implemented for the discovery of the best solution. The calculation of this solution is based on the optimum usage of the network resources, assumed that a minimum quality of service is provided to the user. Moreover, all the messages, required for transferring the access selection data to and from the decision point, are designed on already existing protocols. The goal is to keep simplicity and operability. Furthermore, the selection of the appropriate protocols is justified by the examination of various candidates and the comparison with similar approaches.Finally, the benefits of the proposals of this thesis are validated by the execution of NS-2 simulations. Various scenarios are analyzed, especially covering cases of detecting and solving congestion situations.