Terrorism, radicalization, and (violent) extremism are all contested concepts, referring to specific political labels, ideologies, and related actions. The oldest of the three is “terrorism,” which, as a term, has been around since the late eighteenth century. “Radicalization” is derived from “radicalism” which has its roots in the progressive programs of nineteenth-century political parties that fought, inter alia, for secularism and democracy. “Extremism” is a term first associated with totalitarian movements emerging in the twentieth century in the wake of the First World War. “Violent extremism” is a construct that surfaced only at the beginning of the twenty-first century, referring initially to Islamist ideologies but became subsequently extended to acts of violence by secular groups, especially those on the far right as well. The three concepts are sometimes used wrongly interchangeably, as quasi equivalents. In this chapter some of their characteristics and relationships are explained. Practical and policy implications on countering (violent) extremism, based on the opposite of extremism – moderation – are outlined at the end.