Terrorist and extremist movements have long exploited mass communications technology in pursuit of their political ends. The advent of the internet offers new opportunities. In response, state counter-measures seek to stem the impact of extreme ideologies by a number of tactics. “Positive” measures refer to those online initiatives that seek to make an impact through digital engagement and education and the provision of counter-narratives. “Negative” measures describe those approaches that advocate for, or result in, the deletion or restriction of violent extremist online content and/or the legal sanctioning of its online purveyors or users. More sanctions-based outcomes arise through discretionary state activity such as warnings and counselling of vulnerable individuals, or through disruptive counter-measures such as bans on the giving of lectures or prohibitions on the entry into the country of speakers or the taking down of extremist internet sites. Other measures step over into criminal justice, as when individuals are prosecuted for collecting materials or information (including typically information downloaded from the internet) or for issuing messages which can be construed as direct or indirect incitements to terrorism. This paper will analyse the responses to extremist uses of the internet, with an emphasis upon legal responses – “online laws”.