Scholars have debated the definition of terrorism since the creation of “terrorism” as a field of study. Differentiations between “state” and “non-state” terrorism also abound. Reversely, few have developed formal conceptualizations of counter-terrorism (CT) and its subtypes, state and non-state CT. This chapter helps fill this gap by presenting a formal conceptualization of non-state CT. By doing so, the chapter facilitates scientific studies of CT in all of its guises. To ensure that the conceptualization presented is compatible with a wide range of cases, periods, and places, the definitional method will be more analytical than synthetic. This said, the chapter also uses empirical examples from both the pre- and post-9/11 era. The chapter is divided into three sections: the first section reviews previous attempts to define non-state CT; the second section presents the historical specificity of the terms on which non-state CT is based, such as terrorism and state, but also associated terms, such as civil society and security; the third section presents some subtypes of non-state CT, and discusses their relevance to contemporary debates on “community-based” CT and its effects on civil societies and liberal democracies.