This dissertation explores Thomas Aquinas's understanding of the concept of obligation, and its relationship to law and the morality of human acts, in order to sketch Aquinas's account of "moral obligation." This investigation proceeds through exegesis of central texts in Aquinas's corpus that illuminate his understanding of obligation and its relationship to law and morality of human acts as well as its contrast with the understanding of that same relationship provided by his contemporaries, later interpreters, and future moral philosophers and theologians. The central conclusion reached by this study is that the morality of human acts, principally and primarily, consists in their conformity or contrariety with the command or prohibition of a divine lawgiver and that the binding power proper to law cannot be fully grasped without understanding law (and the act of intellect in the lawgiver by which it is produced) as an evaluative standard of the morality of human acts.Although there are countless studies devoted to Aquinas's account of the natural law, and its relationship to his theory of moral value, few give sustained attention to the distinctive legal character and binding power of the natural law. In part, this neglect stems from an assumption that, because Aquinas insists that reason is the ultimate evaluative standard of the morality of human acts, the goodness or badness of human acts cannot consist, principally and primarily, in conformity or contrariety to a command or prohibition of a divine lawgiver. This dissertation challenges that assumption, clearing the ground for a rapprochement between Aquinas and adherents to divine command theory. Along the way, attention is given to Aquinas's account of the liberum arbitrium of God vis-à-vis the morality of the created order, law's relation to the intellect and will of ruler and ruled, the binding power proper to conscience and its relationship to the law of God, the unique manner in which the natural law binds and is promulgated, the necessity proper to obligation and other kinds of necessity to which human persons might be subject, and the relationship between the law of God, beatitude, and the motivation to be "moral."