This chapter deals with the spectrum of extremism, including far-right, socialist and jihadist movements. It focuses on 'terrorism' as it is generally understood and therefore excludes non-violent groups and state-sponsored actors. The chapter addresses the mechanisms of the 'radical' or 'extremist' online world. It discusses the uses to terrorists – communicative, social, symbolic and operational – of social media and the Internet. The chapter describes the interaction of the online and offline realms, focusing on debates about cyberspace as an initiator, facilitator or 'force multiplier' of extremism in the 'real world'. It assesses online counter-radicalisation efforts to date, categorised in two ways: 'negative' activities which seek to 'inhibit' terrorist movements online, and 'positive' counter-narratives which offer alternative content that challenges radical messages. The online environment can provide safe spaces for debate, organisation and networking, and risks of detection are perceived to be lower than offline.