Our era is marked by a perceived surge in extreme ideas and behaviors, including conspiracy theories, culture wars, religious radicalization, identity politics, ethno-nationalism, and populism. This rise seems linked to tumultuous and disorienting developments such as the restructuring of the international order, the environmental crisis, globalization, and a profound crisis within liberal democracies. “Fanaticism” and “fanatical” denote an uncompromising commitment to an idea or cause, coupled with blind, intolerant, zealous, and self-righteous certainty. These terms, laden with value judgments and emotional connotations, have evolved through history, accumulating conflicting nuances. While often weaponized as polemical labels, they also serve as heuristic tools to comprehend social, religious, and political phenomena. This volume aims to help readers navigate some of the issues that radicalize people today and to chart their intricate historical roots. Authored by scholars with diverse disciplinary backgrounds, the collected essays offer a broad perspective on exploring and conceptualizing the issue of fanaticism.