This paper provides a broad summary of our current understanding of the role of the Internet in relation to extremist and terrorist networks. It covers the role of the Internet in radicalisation; the use of new technologies by extremists looking to organize and/or use violence, whether in a group or alone (including how this is changing the presence of women online); the ways in which the internet has been used as an operation tool by terrorists (to recruit, train, coordinate and communicate); and the range of policy responses, including the emerging priority area of counter-narratives to take on the dominant messages of extremists and challenge their legitimacy.