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This article addresses several challenges related to the evolution toward seamless interworking of wireless LAN and 3G cellular networks. The main objective is to evaluate the conditions and restrictions under which seamless continuity of video sessions across the two networks is feasible. For this purpose, we formulate a number of practical interworking scenarios, where UMTS subscribers with ongoing real-time video sessions hand over to WLAN, and we study the feasibility of seamless continuity by means of simulation. We particularly quantify the maximum number of UMTS subscribers that can be admitted to the WLAN, subject to maintaining the same level of UMTS QoS and respecting the WLAN policies. Our results indicate that the WLAN can support seamless continuity of video sessions for only a limited number of UMTS subscribers, which depends on the applied WLAN policy, access parameters, and QoS requirements. In addition to this study, we do address several other issues that are equally important to seamless session continuity, such as the QoS discrepancies across UMTS and WLAN, the vertical handover details, and various means of access control and differentiation between regular WLAN data users and UMTS subscribers. The framework for discussing these issues is created by considering a practical UMTS/WLAN interworking architecture.