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The key enabling function for seamless mobility and service continuity among a variety of wireless access technologies is the handover. Handovers within the same radio system are addressed by the standardization bodies involved in the development of the corresponding technologies (e.g., 3GPP, 3GPP2, IEEE, DVB), while handovers between heterogeneous systems are managed by protocols developed by the IETF. However, the interoperability between radio access systems that is required to realize the vision of Beyond 3G calls for coordinated actions and integrated solutions combining individual strengths. This article reviews emerging protocols and architectures aiming to support intersystem handovers between nextgeneration wireless systems and presents an optimized handover framework built around the functionality introduced by the IEEE 802.21 standard. Mapping of this framework to the entities of the 3GPP evolved system architecture is discussed and handover procedures involving key entities of this architecture are presented.