Our Master’s thesis is about Vertical handovers in Heterogeneous networks. Due to lot of benefits provided by both the third generation Mobile phone networks and WLANs, it is desirable to integrate both of these networks. While studies specifying generic integration architectures are abundant, little or no studies are dedicated for applications performance over such heterogeneous networks. The purpose of this thesis is to evaluate the performance of integrated UMTS and WLAN network by using two types of standard integrated models proposed by 3GPP(Third Generation Partnership Project) which are open coupling and loose coupling together with network layer mobility management scheme such as Mobile IP. The performance evaluation is carried out for the application i-e FTP, HTTP. We used OPNET 14.5 modular as a simulation tool. We have created a large matrix of performance figures for different network configurations under these two are applications considered. We build the two different scenarios one for loose coupling scheme and other one for open coupling scheme and calculate the different application’s traffic our concluded results shows that page response time is smaller for loose coupling scheme as compare to open coupling scheme although difference is not so much wide, page response time increases with the passage of time because number of users increases as the time passes. Same is In case for FTP, download response time for FTP is smaller in case of loose coupling scheme. We have also calculated the WLAN performance parameters (delay and throughput) and UMTS downlink traffic sent and received. We have discussed in detail architecture and functionalities of UMTS and WLAN, mobile IP, standard interworked UMTS and WLAN models proposed by 3GPP.