In recent years, cellular networks have become open public networks to which end subscribers have direct access. This has greatly increased the number of threats to cellular networks. Though cellular networks have vastly advanced in their performance abilities, the security of these networks still remains highly outdated. As a result, they are one of the most insecure networks today – so much so, that using simple off-the-shelf equipment, any adversary can cause major network outages affecting millions of subscribers. In this chapter, we address the security of cellular networks. We also educate readers on the current state of security of cellular networks and their vulnerabilities. In addition, we outline a cellular network specific attack taxonomy–also called the three-dimensional attack taxonomy. Furthermore, we also discuss the vulnerability assessment tools for cellular networks. Finally, we provide insights as to why cellular networks are so vulnerable and why securing them can prevent communication outages during emergencies.