The fourth generation (4G) of mobile networks will be a technology-opportunistic and user-centric system combining the economic and technological advantages of different transmission technologies to provide a context-aware and adaptive service access anywhere and at any time. Security turns out to be one of the major problems that arise at different interfaces when trying to realize such a heterogeneous system by integrating the existing wireless and mobile systems. Indeed, current wireless systems use very different and difficult to combine proprietary security mechanisms, typically relying on the associated user and infrastructure management means. It is generally impossible to apply a security policy to a system consisting of different heterogeneous subsystems. In this chapter, we first briefly present the security of candidate 4G access systems, such as 2/3G, wireless LAN (WLAN), WiMax, and so forth. In the next step, we discuss the arising security issues of the system interconnection. We namely define a logical access problem in heterogeneous systems and show that both the technology-bound, low-layer and the overlaid high-layer access architectures exhibit clear shortcomings. We present and discuss several proposed approaches aimed at achieving an adaptive, scalable, rapid, easy-to-manage, and secure 4G service access independently of the used operator and infrastructure. We then define general requirements on candidate systems to support such 4G security.