Following the September 11th terrorist attacks in the United States, policy approaches to extremism have mainly focused on understanding the dynamics of religious-based extremism, such as Al-Qaeda and other violent Jihadist/ Islamist groups. Predominantly, the emphasis has been on mapping individual pathways into these particular forms of extreme mobilization. Attacks in Paris, Brussels, and Manchester, as well as in North Africa, Somalia, and Yemen confirm the value of this work in light of the continuing dominance of the security challenges posed by radical Islam, not least in relation to the rise of the so-called Islamic State, or ISIS. The large number of incidents in Europe and elsewhere have resulted in calls for counter-strategies to be modified or expanded, but also for greater resources to be devoted to understanding other forms of extremism, particularly those associated with extreme far right responses or ethnic nationalist ideologies. The economic crisis and its policy responses, along with migration, integration and asylum policies, have affected the relationship between populism and extremism in a fundamental and encompassing manner. The electoral successes of populist, Eurosceptic, and far right parties confirm such tendencies, showing the capacity of extremist discourses to mobilize constituencies against vulnerable groups (e.g., ethnic minorities and immigrants), other countries, and international institutions. Various versions of cultural nationalism have underpinned such mobilization, marked among other things by xenophobia, anti-Semitism, misogyny and Islamophobia, in turn fueling violence. The January 6, 2021 attack at the United States Capitol adds to this picture and lays bare a number of issues related to disinformation, polarization, and the challenge to democratic institutions.This special issue aims to widen the analysis of extremism to account for the unresolved puzzles that continue to plague practitioners, policy makers, and academics alike: