Treatise on Law is Thomas Aquinas' major work of legal philosophy. It forms questions 90–108 of the Prima Secundæ of the Summa Theologiæ, Aquinas' masterwork of Scholastic philosophical theology. Along with Aristotelianism, it forms the basis for the legal theory of Catholic canon law.

The canon law of the Catholic Church is "how the Church organizes and governs herself". It is the system of laws and ecclesiastical legal principles made and enforced by the hierarchical authorities of the Catholic Church to regulate its external organization and government and to order and direct the activities of Catholics toward the mission of the Church. It was the first modern Western legal system and is the oldest continuously functioning system of law in the West, while the unique traditions of Eastern Catholic canon law govern the 23 Eastern Catholic particular churches sui iuris.