Catholic 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 reason. Aquinas's thought sanctions neither an absolute acceptance nor an absolute rejection of the death penalty; for him, this is not a question that admits of absolutes. Like other punishments, the death penalty is a determination made by human reason. Its justification depends on specific historical and cultural circumstances and on the needs of the political community, as well as on the severity of the offense. Killing a guilty person is not intrinsically evil, in Aquinas's view, but it is nonetheless a last resort, when nothing else can be done for the good of the community. It may be that recent Church documents have avoided making use of the Thomistic teaching on the death penalty, even where this could have made their reasoning clearer, for fear that such arguments would be misunderstood, or in order to make a clearer case for forgoing the penalty. If this dissertation contributes to our understanding of what Thomas actually says about CP, it will be helpful in reconciling the thought of John Paul II with the tradition of Catholic thought on capital punishment, as well as in offering a reasonable way for thinking about punishments in general.