This text provides an introduction to Security Studies, the sub-discipline of International Relations that deals with the study of security. War and the threat to use force are part of the security equation, but the prevalence of threats is far-reaching for Security Studies. They encompass dangers ranging from pandemic and environmental degradation to terrorism and inter-state armed conflict. The latter is actually a sub-field of Security Studies and is known as Strategic Studies. This edition examines differing approaches to the study of security such as realism, liberalism, social constructivism, and postcolonialism. It also investigates the deepening and broadening of security to include military security, regime security, societal security, environmental security, and economic security. Finally, it discusses a range of traditional and non-traditional issues that have emerged on the security agenda, including weapons of mass destruction, terrorism, energy security, and health.