Are there objective criteria that we can use to discern if an act of violence constitutes terrorism, or is such labeling always a subjective and political decision? Wherein lies the boundary between domestic versus international terrori and is that a meaningful distinction to make? How do individuals get radicalized, and how do they reach the point of committing violent acts? In this chapter, we tackle these questions (and others) and the issue of terrorism in international security. There are no easy, agreed upon answers to most of them, and terrorism continues to be a highly contested and politically charged concept, while constituting a very real and pressing security threat in many countries around the world. But that is even more reason to look closely at the controversies surrounding its definition, its historical evolution and patterns, and its contemporary manifestations in the twenty-first century as well as approaches to countering terrorism and attempts at international cooperation.