This book introduces fundamental security notions, common threat models, design rationales and basic tools of cybersecurity through the lens of 5G Next Generation Networks to help engineers and students understand the security issues faced by 5G. The author explores core technologies in 5G, highlighting what constitutes the qualitative leap in the 5G transition and why security considerations are different from previous transitions. Next, the book presents a focused discussion of security objectives (confidentiality, integrity, availability, authentication), risk assessment, key cybersecurity design principles (defense-in-depth, cyber kill chain, separation of duties, etc.) and commonly used security technologies (access control, isolated secure execution, and cryptographic protocols for entity authentication, integrity protection and encryption). The book fosters adversarial thinking to assess a security design from the attacker’s perspective rather than from the defender’s, which is essential to robust security designs against unseen threats.