In recent years, the quest for ubiquitous wireless access has led to the early development and deployment of a wide variety of new access networks. In the foreseeable future, the Next Generation Mobile Network (NGMN) tends to integrate multiple access networks to enable service at anytime, anywhere with any technology while maintaining a guaranteed Quality of Service (QoS) for the users. In this book, we contribute to the feasibility of seamless mobility in heterogeneous networks by enhancing different aspects of vertical handover. We validate the related performance by Network Simulator-2 (NS-2). From the results, we can observe that the proposed schemes have the capability to enhance the performance of vertical handover.