The aim of this book is to investigate the interworking of heterogeneous IEEE 802.11 WLANs and UMTS cellular networks. The development of such a network involves many technical challenges which includes network security, seamless VHO, session continuity, consistent quality of service (QoS), network synchronization issues, common authentication, and unified billing. This book focuses on the issue of VHO in the HWNs with the following objectives: 1. To identify a new scheme for a HWN environment. Using transport layer with a conventional mSCTP and packet reordering method. 2. To mitigate the issues of MIP such as additional routing, time of registration and data packet overheads. 3. To develop a new VHO scheme, to ensure seamless handover even when the MN moves at high velocities between cellular UMTS and Wireless LANs networks.