Utilitarianism is a book written by English philosopher and economist John Stuart Mill, considered to be a classic exposition and defense of utilitarianism in ethics. It was originally published in 1861 as a series of three separate articles in Fraser's Magazine before it was collected and reprinted as a single work in 1863. The essay explains utilitarianism to its readers and addresses the numerous criticism against the theory during Mill's lifetime. It was heavily criticized upon publication; however, since then, Utilitarianism gained significant popularity and has been considered "the most influential philosophical articulation of a liberal humanistic morality that was produced in the nineteenth century."

Mill took many elements of his version of utilitarianism from Jeremy Bentham, the great nineteenth-century legal reformer and the propounder of utilitarianism, who along with William Paley were the two most influential English utilitarians prior to Mill. Like Bentham, Mill believed that happiness (or pleasure, which both Bentham and Mill equated with happiness) was the only thing humans do and should desire for its own sake. Since happiness is the only intrinsic good, and since more happiness is preferable to less, the goal of the ethical life is to maximize happiness. This is what Bentham and Mill call "the principle of utility" or "the greatest-happiness principle." Both Bentham and Mill thus endorse "classical" or "hedonistic" forms of utilitarianism.

Although Mill agreed with Bentham about many of the foundational principles of ethics, he also had some major disagreements. In particular, Mill tried to develop a more refined form of utilitarianism that would harmonize better with ordinary morality and highlight the importance in the ethical life of intellectual pleasures, self-development, high ideals of character, and conventional moral rules.

In Chapter 1, titled "General Remarks," Mill notes that there has been little progress in ethics. Since the beginning of philosophy, the same issues have been debated over and over again, and philosophers continue to disagree sharply over the basic starting points of ethics. Mill argues that these philosophical disputes have not seriously damaged popular morality, largely because conventional morality is substantially, though implicitly, utilitarian. He concludes the chapter by noting that he will not attempt to give a strict "proof" of the greatest-happiness principle. Like Bentham, Mill believed that ultimate ends and first principles cannot be demonstrated, since they lie at the foundation of everything else that we know and believe. Nevertheless, he claims, "[c]considerations may be presented capable of determining the intellect," which amount to something close to a proof of the principle of utility.

In the second chapter, Mill formulates a single ethical principle, the principle of utility or greatest-happiness principle, from which he says all utilitarian ethical principles are derived: "The creed which accepts as the foundation of morals utility, or the greatest happiness principle, holds that actions are right in proportion as they tend to promote happiness, wrong as they tend to produce the reverse of happiness. By happiness is intended pleasure, and the absence of pain; by unhappiness, pain, and the privation of pleasure."

Mill then spends the bulk of Chapter 2 responding to a number of common criticisms of utilitarianism. These include charges that utilitarianism:

In response to the charge that utilitarianism is a doctrine fit only for swine, Mill abandons Bentham's apparent view that pleasures differ only in quantity, not quality. He notes that most people who have experienced both physical and intellectual pleasures tend to greatly prefer the latter. Few people, he claims, would choose to trade places with an animal, a fool, or an ignoramus for any amount of bodily pleasure they might thereby acquire. And since "the sole evidence it is possible to produce that something is desirable, is that people do actually desire it," it follows that intellectual pleasures (e.g., the pleasures of friendship, art, reading, and conversation) are higher and more desirable kinds of pleasures than bodily pleasures, and that a rational pursuit of one's long-term happiness requires development of one's higher faculties.

In reply to the objection that there generally is not enough time to calculate how a given act might affect the long-term general happiness, Mill sketches a kind of "two-tier" approach to ethics that accords an important place to moral rules in ethical decision-making. Mill argues that traditional moral rules such as "Keep your promises" and "Tell the truth" have been shown by long experience to promote the welfare of society. Normally we should follow such "secondary principles" without reflecting much on the consequences of our acts. As a rule, only when such second-tier principles conflict is it necessary (or wise) to appeal to the principle of utility directly.