What is fanaticism and why is it an important philosophical topic? In this introductory chapter, I discuss the way in which fanaticism arose as a central philosophical concern in the early modern period. Philosophical discussions of fanaticism focus on psychological, epistemic, and behavioral dimensions of fanatics. The fanatic displays psychological peculiarities; epistemic defects; and potentially problematic behavioral tendencies. I discuss the ways in which different philosophers have offered different accounts of these three features; offer a brief defense of my own account of fanaticism; and highlight some key questions about fanaticism. I close with an overview of the essays in this volume.