This paper presents the results from exploratory primary research into the role of the internet in the radicalisation of 15 terrorists and extremists in the UK. The 15 cases were identified by the research team together with the UK Association of Chief Police Officers (ACPO) and UK Counter Terrorism Units (CTU). The research team gathered primary data relating to five extremist cases (the individuals were part of the Channel programme, a UK government intervention aimed at individuals identified by the police as vulnerable to violent extremism), and ten terrorist cases (convicted in the UK), all of which were anonymised. The team conducted interviews with the Senior Investigative Officers (SIOs) involved with the terrorists and Channel participants, and investigated the individuals’ online behavior from data recovered by the police directly from the individuals’ computers. The team then conducted a literature review and developed a number of hypotheses or assertions found in the literature on the role of the internet in the process of radicalisation. These hypotheses were tested using primary data from the above mentioned 15 cases.