This chapter focuses on the major sources of moral definition and authority within Roman Catholicism. It extends from the early Middle Ages to the First Council of the Vatican, which convened in 1869. During most of this lengthy period, canon law and moral theology were the major sources of Roman Catholic medical ethics. Canon law consisted of private compilations of canons of councils, papal rescripts, and episcopal statutes, often inconsistent with each other. The influence of canon law on the development of Roman Catholic medical ethics has been profound, especially in three respects. The chapter describes the role of the magisterium as a source of moral definition. The Church's tradition has constantly upheld, and the magisterium has consistently proposed the moral truth of the absolute inviolability of innocent human life. Within the Church the magisterium is ordinarily exercised in teaching moral principles and precepts through catechesis and preaching.