In this essay, Walter Nicgorski describes the formative influence of Cicero on natural law theory. Cicero, an ancient Roman philosopher and statesman, set out one of the earliest expositions of natural law, thus providing intellectual inspiration for many medieval and modern thinkers, including the American Founders. Drawing on both Greek philosophy and the Roman juridical tradition, Cicero uses probing arguments to demonstrate the existence of a natural law that fulfills both human nature and the divine order of the universe. Specifically, he grounds both the knowledge and implementation of natural law in the practice of virtue. This gives him the ability to confront challenges to the very idea of natural law, and to show how it can be useful in making practical decisions, such that familiarity with his thought is essential for any contemporary student of the subject.