We live in a digital era. In the UK alone 85 per cent of homes have internet access. As society increasingly embraces the internet, so opportunities for those wishing to use it for terrorism have grown. The internet offers terrorists and extremists the capability to communicate, collaborate and convince. In recent years, European policymakers, practitioners and the academic community have begun to examine how the internet influences the process of radicalisation: how a person comes to support terrorism and forms of extremism associated with terrorism. Many of the policy documents and academic literature in this area focus on online content and messaging, rather than exploring how the internet is used by individuals in the process of their radicalisation. The reason for this focus is relatively straightforward. Gaining access to terrorists (those convicted under UK terrorism legislation) or extremists (identified by the police and multi-agency partners based on an assessment of risk) is extremely difficult. Obtaining primary data relating to these individuals’ cases held in court records or by the police is labour-intensive and a logistical challenge. However, empirical research is needed in order to rigorously test assertions about radicalisation and formulate evidence based approaches to addressing challenges associated with radicalisation. In order to begin to address this gap and develop the evidence base in the field, this study is based on primary data drawn from a variety of sources: evidence presented at trial, computer registries of convicted terrorists, interviews with convicted terrorists and extremists, as well as police senior investigative officers responsible for terrorist investigations. The sample size is small: a symptom of both the limited number of individuals willing to speak with researchers in this field, and the challenge of collecting data on such a sensitive topic where information in the public domain is limited.