In this magisterial work, Joseph O'Callaghan offers a detailed account of the establishment of Alfonso X's legal code, the Libro de las leyes or Siete Partidas, and its applications in the daily life of thirteenth-century Iberia, both within and far beyond the royal courts. O'Callaghan argues that Alfonso X, el Sabio (the Wise), was the Justinian of his age, one of the truly great legal minds of human history. Alfonso X, the Justinian of His Age highlights the struggles the king faced in creating a new, coherent, inclusive, and all-embracing body of law during his reign, O'Callaghan also considers Alfonso X's own understanding of his role as king, lawgiver, and defender of the faith in order to evaluate the impact of his achievement on the administration of justice. Indeed, such was the power and authority of the Alfonsine code that it proved the king's downfall when his son invoked it to challenge his rule. Throughout this soaring legal and historical biography, O'Callaghan reminds us of the long-term impacts of Alfonso X's legal works, not just on Castilian (and later, Iberian) life, but on the administration of justice across the world.