Is terrorism successful? This oft-asked but infrequently answered question is at the heart of understanding terrorism's use. Effective policy and practice rest on a clear appreciation of how terrorism succeeds and fails. This paper identifies and reviews the literature on terrorism's effectiveness and considers its strengths and weaknesses. To organise the review, a multi-level framework for conceptualising the goals of terrorism and its enactors is presented. Crucially, this includes the perspective of both sides of the conflict dyad, and considers the tactical, organisational, strategic and ultimate goals of extremist groups. This speaks to the need for a comprehensive framework within which to situate the question of terrorism's goals, and aims to assess the state of the knowledge with respect to terrorism's efficacy.