Why and how do individuals residing in relatively peaceful and affluent Western societies come to embrace extremist ideologies that emanate from distant places? We summarize the most recent empirical literature on the causes and dynamics of radicalization, and evaluate the state of the art in the study of Islamist homegrown extremism in the West. We propose a theoretical synthesis based on four factors that come together to produce violent radicalization: personal and collective grievances, networks and interpersonal ties, political and religious ideologies, and enabling environments and support structures. We propose adopting a “puzzle” metaphor that represents a multifactor and contextualized approach to understanding how ordinary individuals transform into violent extremists. We concluded with three recommendations to strengthen the empirical foundations of radicalization studies.