Following Aristotle, Aquinas posited three virtues of the speculative intellect: science, wisdom, and understanding. Aquinas’s accounts of the virtues of science and wisdom have received a great deal of attention, as has his notion of intellectual virtue in general. But understanding as an intellectual virtue has received almost no attention at all. Part of the difficulty stems from Aquinas himself: Aquinas uses intellectus broadly, and typically without specifying whether he means to refer to our natural habitual knowledge of first principles, the act of understanding, or a developed virtue. At the same time, however, Aquinas clearly considers the virtue of understanding to be both important in its own right and fundamental to the virtues of science and wisdom. This chapter seeks to examine understanding as an intellectual virtue in Aquinas and to propose a hypothesis about what, given Aquinas’s account, it would mean to develop the intellectual virtue of understanding. I will argue that Aquinas’s account implies that we can develop the virtue of understanding only insofar as we can come to increase what we habitually know and thereby pave the way for our understanding to operate more readily and more effectively. In this way, the virtue of understanding is importantly different from the other intellectual virtues.