As a result of rapid progress in research and development, today's wireless world exhibits several heterogeneous communication networks, such as cellular networks, satellite networks, wireless local area networks (WLAN), mobile ad hoc networks (MANET), and sensor networks. These networks are complementary to each other. Hence, their integration can realize a unified wireless system that has the best features of the individual networks. This has spurred much research interest in designing integrated next-generation of wireless systems (NGWS). While existing wireless networks have been extensively studied individually, the integrated wireless system brings new challenges in architecture design, system management, and protocol design. The different wireless networks use different communication technologies and are based on different networking paradigms. Therefore, it is challenging to integrate these networks such that their heterogeneities are hidden from each other and a harmonious inter-operation among them is achieved. The objective of this research is to design a scalable, secure, and robust architecture and to develop seamless mobility management protocols for NGWS. More specifically, an architecture that integrates the heterogeneous wireless systems is first proposed for NGWS. Next, a cross-layer (Layer 2 + 3) handoff management protocol is developed for NGWS. Afterward, analytical modeling is developed to investigate the handoff performance of the existing mobility management protocols for different types of applications. Finally, a framework for multi-layer mobility management is developed to support the seamless handoff support to all types of applications in NGWS.