Political extremists and terrorists are increasingly using the internet as an instrument for radicalisation and recruitment. What can be done to counter their activities? The Challenge of Online Radicalisation examines the different technical options for making ‘radical’ internet content unavailable, concluding that they all are either crude, expensive or counter-productive. It sets out a new, innovative strategy which goes beyond ‘pulling the plug’, developing concrete proposals aimed at: Studying how to deter the producers of extremist materials, Giving users the keys to self-regulate their online communities, Reducing the appeal of extremist messages through education, Promoting positive messages