The aim of the present literature review is to provide a comprehensive overview of the empirical literature on radicalisation leading to extremism. Two research questions are asked: (1) Under what conditions are individuals receptive to extremist groups and their ideology? (2) Under what conditions do individuals engage in extremist acts? A theoretical framework is used to structure the findings. A systematic literature search was conducted including peer-reviewed articles containing primary qualitative or quantitative data. A total of 707 empirical articles were included which used quantitative or qualitative research methods. The findings clearly indicate that no single factor in itself predicts receptiveness to extremist ideas and groups, or engagement in violent behaviour. Rather, factors at different levels of analysis (micro-, meso- and macro-level) interplay in the radicalisation process.