This dissertation explores how Britons used their experiences raising five of the most prolific European animals -- cows, sheep, horses, pigs, and dogs -- to understand, implement, and question the enslavement of humans in America between 1550 and 1815. Expanding the historiographical boundaries of slave and animal histories, I argue that whites put Africans into bondage as an outgrowth of their divine sanction to "improve" nature, to turn wild and unproductive beasts into tame and useful ones. This process was far from static. As farmers and agricultural improvers developed new, more rigorously mercantile, ways of relating to livestock at the center of the British Empire, they shaped and were shaped by a growing commodification of slaves on the peripheries. Beginning in the seventeenth century and continuing with breeding experiments with sheep in the eighteenth, herd animals in Britain were transformed from individual servants on the farm into aggregates, vast collectives of living meat. Horses and dogs saw their conditions improve, but they too were increasingly bred for profit and specialized labor. While colonists were slow to adopt British innovations as they related to animals, they supported and appropriated them in their interactions with Africans. As early as the sixteenth century, explorers and artists characterized Africans as wild and unproductive animals whom whites had a duty to make useful and profitable. Colonial officials, first in Barbados and then in the Chesapeake and Lowcountry, extended these arguments to the legal realm, often placing servants and slaves in the same categories as livestock. And planters increasingly treated slaves as equivalent units of financial production, replaceable cogs in massive and lucrative plantation machines. As Britons on both sides of the Atlantic saw the improvement of nature as a mercantile rather than moral endeavor, and thus treated slaves as commodities, eighteenth- and early nineteenth-century century humanitarian reformers came to criticize slavery and animal cruelty alike.