The next generation of mobile/wireless communication system is expected to include heterogeneous broadband wireless networks that will coexist and use a common IP core to offer a diverse range of high data rate multimedia services to end users with contemporary mobile devices. The mobile devices will be equipped with multiple network interfaces since the networks have characteristics that complement each other. This requires the provision of seamless vertical handover. In this paper, a probability driven handover scheme which uses the mobile's speed (SPHO) is developed to support seamless and adaptive handover management between WLAN and UMTS. Extensive simulation experiments show that this scheme effectively achieves seamless handover between heterogeneous networks and guarantees quality of communication during handover.