This review presents the existing research on the role of the Internet in radicalization processes. Using a systematic literature search strategy, our paper yields 88 studies on the role of the Internet in a) right-wing extremism and b) radical jihadism. Available studies display a predominant interest in the characteristics of radical websites and a remarkable absence of a user-centred perspective. They show that extremist groups make use of the Internet to spread right wing or jihadist ideologies, connect like-minded others in echo chambers and cloaked websites, and address particularly marginalized individuals of a society, with specific strategies for recruitment. Existing studies have thus far not sufficiently examined the users of available sites, nor have they studied the causal mechanisms that unfold at the intersection between the Internet and its users. The present review suggests avenues for future research, drawing on media and violence research and research on social identity and deindividuation effects in computer-mediated communication.