Luis de Molina (1535-1600) was the first scholastic doctor to legitimize the practice of money lending as a career. His De Iustitia et Iure offers a thorough description of trade practices of the vibrant economies of Portugal and Spain in the Sixteenth Century. This detailed analysis allows him to provide a moral assessment of these practices. His treatise is a capital example of how a deep commitment to received tradition and to contemporary economic issues can advance economic science and perfect moral theology through a better understanding of reality. This book shows how threads of field research, economic reflection, natural law tradition, casuistry and the quest for justice may weave together to form a major work of Catholic moral theology.