Contemporary philosophers use the expression "weakness of will" when they discuss the inability to act on one's knowledge, yet Aquinas speaks primarily about incontinentia ('incontinence'), which in turn is the common Latin translation of Aristotle's akrasia. This chapter deals with various aspects of Aquinas's account of incontinence in his commentary on the Nicomachean Ethics. Aquinas frequently contrasts incontinence with temperance and its opposite vice, intemperance. In contrast to "weakness", incontinence is a specific "transitory vice" related to pleasures and desires associated with touch. Aquinas's account of incontinence is a subtle attempt to develop Aristotle's teachings. In going beyond Aristotle, Aquinas not only tries to clarify the moral psychology behind the phenomenon of incontinence and to situate it with respect to wider psychological views, and to make efforts to connect Aristotle with a somewhat different philosophical tradition. Aquinas's treatment of incontinence offers some important philosophical guidelines, especially to his contemporaries.