A vision of future wireless networks is the coexistence of multiple access network technologies. Different access networks will coexist and the Internet Protocol will be the glue. To fulfil this vision, one of the greatest challenges is to develop sophisticated policies for vertical handovers. When and in what way the handovers are performed, affects the performance of the mobile services. Traditional handover policies are based on received signal strength comparison. To be able to support vertical handover, policies have to consider parameters such as capacity, utilization cost, power consumption, security and QoS guarantees. This paper describes a proposal for a policy-based vertical handover model for heterogeneous networks. Analysis, design, implementation and evaluation of "SEMO", a policy-based vertical handover system, are presented.