This thesis addresses the relationship between anti-Judaism and the emergence of the Spanish state in the late medieval period. It examines the way anti-Judaism shaped Spanish attitudes and institutions in the 15th century. In particular, it explains the development of anti-Jewish sentiment which led to the anti-Jewish riots of 1391, and the consequent mass conversions and large new population of conversos. These conversos posed a challenge for Spanish society, with the Inquisition being the principal way in which that challenge was met. New ideas of what it meant to be Spanish were developed as a result of the focus on converso communities and the attempt to reckon with the relationship between Old Christians and this large (and often affluent) community, with blood purity and racialism becoming important new elements in the Spanish identity. The obsession with purity of blood and orthodoxy of religion culminated in the expulsion of the Jews in 1492 and the continued investigation and persecution of conversos, even those who were several generations removed from conversion and from Judaism. As part of this analysis, the thesis formulates a new taxonomy for understanding anti-Judaism in Spain prior to the 15th century, dividing it into elitist and populist traditions. It was the wall between these two traditions, this thesis argues, which allowed the Spanish state to satisfy its desire to articulate and enforce a Christian identity while simultaneously finding both a conceptual and a real place in Christian Spanish life for Jews and Jewish communities. The thesis also considers the unique figures of Ferdinand and Isabella and the unique circumstances of their reign (i.e. the conquest of Granada and the completion of the Reconquista in Iberia) and how these immediate conditions led to the breakdown of the old policy of monarchical protection of Jews and the birth of a new and radically anti-Jewish (and, in some ways, even anti-Semitic) policy.