Lying behind the recent Counter-Terrorism and Security Act 2015 is the phenomenon of foreign terrorist fighters which has sparked international and national attention. The 2015 Act deals with many facets of counter terrorism legislation, but its two principal measures are singled out for analysis and critique in this paper. Thus, Part I of the Act seeks to interdict foreign terrorist fighters by preventing suspects from travelling and dealing decisively with those already in the UK who pose a risk. Part V of the Act implements the second, broader aspect, of legislative policy, reflecting the UN emphasis on ‘Countering Violent Extremism’, through the statutory elaboration and enforcement of the ‘Prevent’ element of the long-established Countering International Terrorism strategy, which aims to stop people becoming terrorists or supporting violent extremism. These measures are explained in their policy contexts and set against criteria of effectiveness, personal freedom, and accountability.