This article assesses the use of proscription powers as a tool for countering terrorism, using the United Kingdom as a case study. The article begins with a brief overview of the United Kingdom’s current proscription regime. It then situates this in historical context, noting the significant recent increase in proscribed groups and the predominance of ‘Islamist’ organisations therein. The article then critiques proscription on four principal grounds. First, in terms of the challenges of identifying and designating proscribed groups. Second, we highlight the considerable domestic and transnational politicking that surrounds proscription decisions. Third, we assess the normative importance of protecting scope for political resistance and freedoms of expression and organisation. And, fourth, we question the efficacy of proscription as a counter-terrorism tool. The article concludes by arguing that proscription’s place in contemporary security politics should be heavily safeguarded given these challenges, before pointing to specific policy recommendations to this end.