Extremism has recently been described as one of the most pressing problems in contemporary society (Litter & Lee, 2023). The burst of extremism in today’s world has happened as the hodgepodge of different ideologies replaced the hegemonic ideological blocks of the Cold War (Ugarriza, 2009) and the rise of digital technologies catalyzed this change (Litter & Lee, 2020). In order to propose policies or interventions to reduce or eliminate extremism, it is necessary to understand its causes. What is clear is that extremism results from a multitude of causes, not just one. In this volume, we will focus on one specific contributor which has received relatively little attention in the past: social exclusion.

Norwegian terrorist Anders Breivik was reported to have been excluded throughout his life, much like German extremist Fabian D. In many cases of terrorism like these, social exclusion is found in the life course of terrorists. This chapter therefore explores the relationship between exclusion and terrorism in greater depth. First, central concepts are defined and explained. The chapter then focuses on theoretical and, in particular, empirical findings on the exclusion–terrorism link. According to these findings, there is a causal relationship between exclusion and a terrorist mindset. However, the link between exclusion and terrorism is probably not direct and instead works indirectly, through processes such as deprived needs and higher social susceptibility. Ultimately, exclusion is likely to act as an early cognitive opener for terrorism via its individual- and group-level consequences. A corresponding exclusion–terrorism model is postulated. The chapter concludes with directions for future research and practical implications regarding the exclusion–terrorism link.