One motivation of mobile Next-Generation Networks (NGN) is the ubiquitous computing abilities, which provide automatic handovers for any moving computing devices in a globally networked environment. The vertical handover is defined as the process of changing from one technology to another. For example, the data services are switched to Wireless Local Area Networks (WLAN) from the previously attached Universal Mobile Telecommunication System (UMTS) networks. Identity Management is considered important for vertical handovers for managing continued access to network resources. In this paper, we address several aspects of identity management requirements in the vertical handovers between UMTS and WLAN.