Sources of Catholic moral theology include both the Old Testament and the New Testament, and philosophical ethics such as natural law that are seen as compatible with Catholic doctrine. Moral theology was mostly undifferentiated from theology in general during the patristic era, and is found in the homilies, letters and commentaries on Scripture of the early Church fathers. During the Middle Ages, moral theology developed in precision and scope through scholasticism. Much of the Catholic Church's current moral theology, especially regarding natural law, is based in the Summa Theologica by St. Thomas Aquinas, which is regarded as one of the best treatises of Catholic moral theology.
Contemporary Catholic moral theology is developed by acts of the Magisterium, by the Pope, other bishops, and by the works of lay Catholic moral theologians, which include magisterial teachings, as well as (in some matters) theological opinions. Examples of Catholic moral theologians include St. Alphonsus Liguori (author of Moral Theology), Germain Grisez (author of The Way of the Lord Jesus) and John Finnis (author of Natural Law and Natural Rights). Moral theology tends to be advanced most authoritatively through official statements of doctrine, such as papal encyclicals, which are based on the dogmatic pronouncements of Ecumenical Councils (e.g., Vatican II), Sacred Scriptures, and Sacred Tradition. In addition, moral theologians publish their own works and write in a variety of journals devoted in whole, or in part to moral theology. These scholarly journals are helpful in making the theology of the Church more clear and accessible to others, and serve as a forum in which scholarly discussion of understanding and application of issues occurs. However, these journals per se do not add or remove anything from Catholic teaching.
The curriculum for formation of priests commonly includes required and elective courses in Catholic moral theology.