The topic of my dissertation is the hedonic calculus. The hedonic calculus presupposes that pleasure and pain come in amounts amenable to addition, subtraction, and aggregation operations. These operations are ones that utilitarianism and related normative ethical theories treat as central to moral phenomena. The first chapter is an introduction to the problem--in it, I explain what the hedonic calculus is, why it is important, and why it has recently come under disfavor. The second chapter explores the nature of hedonic phenomena, arguing that pleasure and pain are propositional attitudes; they are not feelings or feeling-tones, nor are they fundamentally a matter of desire or motivation. The third and fourth chapters concern the nature of quantitative phenomena. I argue that quantities are determinate properties whose determinables enter into greater-than, less-than , or equal-to relationships that are homomorphic with the structure of the real number line. These structures include the presence of a unique order; the possibility of equal intervals; and a natural, non-arbitrary zero point. The fifth chapter is a defense of the thesis that there are exactly three relations of quantitative comparison from a recent attack. I argue that apparent instances of being on a par are in fact instances of vagueness or complexity and do not threaten the truth of the trichotomy thesis. The sixth chapter addresses arguments that pleasure and pain fail to meet these formal conditions. One argument stems from the observation that pleasures and pains are essentially transient and ephemeral. Another argument proceeds from the observation that interpersonal comparisons of pleasure and pain appear to be impossible. A third, closely related to the first two, argues that pleasure and pain are too heterogeneous to be quantitative. A final argument holds that, for a variety of reasons, the various mathematical operations constitutive of quantitativeness are not applicable to pleasure or pain. I argue that each argument suffers from at least one fatal flaw. Some arguments crucially involve a failure to distinguish between the existence of a quantity and our ability to perform reliable measurements of it. Others involve false analogies between hedonic and other sorts of phenomena. The seventh chapter addresses arguments in favor of the legitimacy of the calculus. Many philosophers, such as Bentham, Mill, Ross, and Plato, seem to assume, without any serious argument, that pleasure and pain are quantitative. Others attempt to argue that, for example, two people enjoying something must involve more pleasure than if just one person were enjoying it. A third type of argument rests on the claim that it is sometimes rational to be indifferent between a longer lasting, less intense episode of pleasure and a more intense but shorter episode. This suggests that since duration is clearly quantitative, intensity must be, too, because if it were not, there could be no equivalence between the brief but intense and the lengthy but mellow episodes. Several of these arguments are unsound or question-begging. Finally, I present an argument in favor of the hedonic calculus that I endorse, along with possible objections and my replies. I begin by acknowledging the central point of the "hedonic trade-offs" argument, and then proceed, via reductio , by pointing out the various absurd or unpalatable consequences of the denial of the thesis that pleasure and pain are quantitative.