This dissertation studies the role of slave pain in shaping moral evaluations of slavery. In the early eighteenth century, few people doubted slaves had unhappy lives, but misery seemed inevitable, and heaven adequate compensation. Attitudes toward pain gradually shifted from acceptance to repugnance. Criticisms of slaveholding cruelty emerged from two distinct sources, both concerned more with white morality than black pain: Quakers exalting martyrdom, and other Protestants and moral philosophers stressing sympathy and benevolence in moral formation. Pain soon grounded antislavery rhetoric. Antislavery pamphlets and books dwelt, in excruciating detail, on the physical torments of slaves.  But the revulsion against pain influenced slavery's defenders, who argued that cruelty was not essential to the system and that, without it, slavery was not only just, but humane. The colonization movement nurtured both pro- and antislavery rhetoric. It claimed that slavery was cruel, but so was American freedom, offering only starvation, suffering, and prejudice. As colonization splintered, it pitted pro- and antislavery polemicists against each other, both claiming true sympathy for blacks. Antislavery attacks inspired proslavery polemicists to counter that slavery humanely provided food, clothing, shelter, and security. Proslavery claims, in turn, pushed abolitionists to prove slavery's gruesome cruelties, while they simultaneously feared that focusing on individual cases of cruelty obscured the more systematic evil of slavery. The "humanitarian" argument vexed abolitionists, raising questions about the relative importance of physical versus mental pain, the prudence of highlighting cruelty, and the moral role of emotions, imagination, and conscience. Pro- and antislavery ideologies were not self-contained, but took shape in a fierce debate over slavery and the meaning of humanitarianism; exigencies of the debate helped push people with similar moral commitments to widely divergent conclusions.  Despite recognizing the revolution in views of pain, historians have paid little analytical attention to pain in the slavery debate, leaving an unfortunate impression that concern with pain flowed directly into what we think of as humanitarianism. Assuming pain naturally served antislavery obscures the complexity of moral claims about pain. Recognizing this contestation and pain's ability to serve contradictory purposes illuminates our understanding of its role in morality.