The role played by the internet in processes of radicalisation is a pervasive subject in discussions of violent extremism, both in the circles of policymakers, researchers, and the media. While the fact that the internet plays a role is largely accepted as a truth, few studies have sought to examine how the mechanisms behind online-based radicalisation work. The thesis asks to what extent and in what ways the internet and social media influenced the radicalisation process of extreme-right lone actors who have carried out or attempted to carry out large-scale terrorist attacks in Western democracies. Understanding radicalisation as a process, the focus on the analysis is on the underlying mechanisms with a potential to impact radicalisation on the internet. In the absence of a theoretical model of online radicalisation, I identified six mechanisms seen as particularly relevant for explaining the phenomenon of online radicalisation. In order to investigate these mechanisms, I have chosen to combine comparative case study and process tracing. I analysed three cases of radicalisation into extreme-right terrorism where the internet was used, investigating the ways in which the internet impacted on the radicalisation trajectories of all three individuals. In the analysis, I demonstrate that the impact of the internet on radicalisation should neither be overstated nor understated; radicalisation is impacted by both offline and online mechanisms. My findings show that the internet played a role in facilitating radicalisation through making extreme ideology and information accessible, as well as amplifying group polarisation and legitimising extreme ideology and political violence through echoing. However, it is not effective on its own. Offline aspects such as pre-existing vulnerabilities and isolation in the form of detachment from offline social milieus are found to be important, likely because they provide the locus for the individual to seek information and belonging in alternative online milieus where communication is voluntary, and they are in control.