This dissertation considers conceptions of blood during a critical period in which religious differences between Iberian populations were reduced to the doctrine of blood purity (limpieza de sangre), which many scholars cite as the beginnings of a modern conception of race. Blood purity laws, disseminated throughout Spain in the sixteenth century, differentiated converted Jews and Moors (conversos and moriscos) from majority Christians, restricting Church and some government posts to those who could prove “blood purity.” This represented a critical transition in European thought, from conceiving of divisions between “peoples” as multiple to a taxonomic system based on biology that hardened difference to a single, immutable term. Drawing on medical, art, and cultural history as well as literary studies, this dissertation explores “blood” as material and metaphor in early modern Spain in order to trace how Castilian national, imperial, and theological concerns were staged symbolically on the body. Although blood became a term of identity in this period, crucial to the place of the early modern Spanish subject in society, notions of “blood purity” mapped onto to an already complex symbolism of blood in medicine, religion, and social ideology, thus troubling its role in classifying subjects in a racial hierarchy as pure or impure. This dissertation helps illuminate the early modern debate about “blood purity,” illustrating a lively resistance to the suppression of conversos and expulsion of moriscos. Literary works addressed include Miguel de Cervantes’s Numancia, Pedro de Calderón’s El médico de su honra, and love sonnets about bleeding women.