The author presents a necessarily brief summary of Catholic Social Teaching (CST) regarding immigration, featuring especially Pius XII's much neglected apostolic constitution Exsul familia. He also sets out some of the philosophical presuppositions of CST as it pertains to immigration. These presuppositions are to be found, he maintains, especially in the writings of Aristotle and Thomas Aquinas. He then examines in some detail Francisco de Vitoria's ideas regarding immigration, based as they are upon Aristotelian and Thomistic principles. Finally, he offers answers to questions that have arisen over the course of the essay.