The global system for mobile communications (GSM) is the most popular standard that implements second generation (2G) cellular systems. 2G systems combined with general packet radio services (GPRS) are often described as 2.5G, that is, a technology between the 2G and third generation (3G) of mobile systems. GPRS is a service that provides packet radio access for GSM users. This chapter presents the security architecture employed in 2.5G mobile systems focusing on GPRS. More specifically, the security measures applied to protect the mobile users, the radio access network, the fixed part of the network, and the related data of GPRS are presented and analyzed in detail. This analysis reveals the security weaknesses of the applied measures that may lead to the realization of security attacks by adversaries. These attacks threaten network operation and data transfer through it, compromising end users and network security. To defeat the identified risks, current research activities on the GPRS security propose a set of security improvements to the existing GPRS security architecture.