The integration of heterogeneous mobile/wireless networks using an IP-based core network materializes the beyond third generation (B3G) mobile networks. Along with a variety of new perspectives, the new network model raises new security concerns, mainly, because of the complexity of the deployed architecture and the heterogeneity of the employed technologies. In this chapter, we examine and analyze the security architectures and the related security protocols, which are employed in B3G networks focusing on their functionality and the supported security services. The objectives of these protocols are to protect the involved parties and the data exchanged among them. To achieve these, they employ mechanisms that provide mutual authentication as well as ensure the confidentiality and integrity of the data transferred over the wireless interface and specific parts of the core network. Finally, based on the analysis of the security mechanisms, we present a comparison of them that aims at highlighting the deployment advantages of each one and classifies the latter in terms of: (1) security, (2) mobility, and (3) reliability.