Thesis advisor: Arthur MadiganCatholic moral philosophers and theologians for centuries used Thomas Aquinas's defense of the death penalty as a point of reference in defending the state's right to execute. Recent Church documents such as Evangelium Vitae, however, seem to take a different approach to the question than Aquinas did. In secular contemporary treatments of the death penalty, Aquinas's account is often caricatured or simply overlooked. One of the reasons for this is the lack of a thorough treatment of the death penalty in the thought of Aquinas. This dissertation seeks to address that deficiency. I present Aquinas's account of capital punishment as an example of determining civil punishments through the exercise of practical reas...