Why do people persist in commitments that threaten their happiness, security, and comfort? Why do some of our most central, identity-defining commitments seem to resist the effects of reasoning and critical reflection? Drawing on real-life examples, empirical psychology, and philosophical reflection, Paul Katsafanas argues that these commitments involve an ethical stance called devotion, which plays a pervasive—but often hidden—role in human life. Devotion typically involves sacralizing certain values, goals, or relationships. To sacralize a value is to treat it as inviolable (trade-offs with ordinary values are forbidden), incontestable (even contemplating such trade-offs is prohibited), and dialectically invulnerable (no rational considerations can disrupt the agent's commitment to the value). Philosophy of Devotion offers a detailed philosophical account and defense of these features. Devotion and the sacralization of values can be reasonable; indeed, a life involving meaningful, sustained commitment depends on these stances. Without devotion, we risk an existential condition that Katsafanas describes as normative dissipation, in which all of our commitments become etiolated. Yet devotion can easily go wrong, deforming into the individual and group fanaticism that have become pervasive features of modern social life. Katsafanas provides an alternative to fanaticism, investigating the way in which we can express non-pathological forms of devotion. We can be devoted through affirmation and through what Katsafanas calls the deepening move, which treats the agent's central commitments as systematically inchoate. Each of these stances enables a wholehearted form of devotion that nevertheless preserves flexibility and openness, avoiding the dangers of fanaticism on the one hand and normative dissipation on the other. But this is inevitably a fragile and precarious achievement: affirmation can slide into a focus on rejecting what isn't affirmed, and the deepening move can ossify into rigidity. Only the perpetual quest to maintain a form of existential flexibility, which may require oscillation between affirmation and deepening, can stave off these dangers