The use of the Internet, including social media, by violent extremists and terrorists and their supporters has been a source of anxiety for policymakers and publics for a number of years. This is based on the idea that there is a connection between consumption of and networking around violent extremist and terrorist online content and adoption of extremist ideology (i.e. so-called ‘online radicalisation’) and/or recruitment into violent extremist or terrorist groups or movements and/or attack planning and preparation and/or, ultimately, engagement in violent extremism and terrorism. Concerns have been raised, in particular, regarding easy access to large volumes of potentially influencing violent extremist and terrorist content on prominent and heavily trafficked social media platforms.