Despite widespread conceptual ambiguity, there is a coherent logic underpinning how we speak about terrorism, this book argues. What distinguishes terrorism is its underground nature which, unlike other forms of violence, requires no control over territory; perpetrators must act covertly, and attacks must be temporally condensed unlike prolonged battles or occupations. Understanding terrorism as underground violence helps clarify traits often associated with it: the ‘weapon of the weak’ and a form of extreme asymmetric warfare. The book’s first part is conceptually and analytically driven: Chapter 1 addresses what constitutes political violence and what does not qualify as terrorism, while Chapter 2 presents the core claim that terrorism is underground political violence, acknowledging prior scholarship but refining this idea. The authors also explore international terrorism, arguing that once armed groups operate beyond national borders, their actions invariably become terrorist due to the necessity of secrecy. They further distinguish state terrorism from state repression, defining the former as covert operations against rivals. Chapter 3 draws empirical implications from their theory, using quantitative analysis. Their first proposition is that greater asymmetry between states and armed groups increases the likelihood of underground activity when political violence is used. The second contends that clandestine groups are constrained to tactics consistent with their hidden status, unlike groups with territorial control that can choose their tactics. Chapters 4 and 5 develop narrative case studies – Tupamaros, ETA, Shining Path, the Israeli–Palestinian conflict, and ISIS – demonstrating variation in territorial control and clandestinity. The authors begin from a crucial premise: conceptual clarity is essential and has theoretical implications. They offer an original and compelling contribution to a still unsettled debate.