Overview: Proscription laws criminalising membership of extremist organisations have a long and chequered history, stretching back as far as ancient Rome of 82 BC. In their contemporary guise, proscription laws in Australia, Canada, the UK and US have largely been calibrated to tackle the threat of Islamist violent extremism (and, for Australia, Canada and the US, criminality associated with motorcycle gangs). Detractors of proscription laws have argued that the powers have been used arbitrarily to suppress legitimate state dissent, protect state interests and represent politicised policing. Proponents of the powers, by contrast, argue that proscription powers are a necessary and proportionate response to the very real terrorist threat to the state and the public. The evidence remains unclear on whether proscription powers actually strengthen the arm of policing and security agencies or, indeed, curb terrorist activity associated with the proscribed organisations. This Chapter undertakes a comparative review of the the proscription laws and powers across Australia, Canada, the UK and US and asks: how effective are proscription laws in countering terrorism?