International security is an ambiguous concept – it has many meanings to many people. Without an idea of how the world works, or how security is defined and achieved, it is impossible to create effective policies to provide security. This textbook clarifies the concept of security, the debates around it, how it is defined, and how it is pursued. Tracking scholarly approaches within security studies against empirical developments in international affairs, historical and contemporary security issues are examined through various theoretical and conceptual models. Chapters cover a wide range of topics, including war and warfare, political violence and terrorism, cyber security, environmental security, energy security, economic security, and global public health. Students are supported by illustrative vignettes, bolded key terms and an end-of-book glossary, maps, box features, discussion questions, and further reading suggestions, and instructors have access to adaptable lecture slides.