In ethical philosophy, utilitarianism is a family of normative ethical theories that prescribe actions that maximize happiness and well-being for the affected individuals. In other words, utilitarian ideas encourage actions that lead to the greatest good for the greatest number. Although different varieties of utilitarianism admit different characterizations, the basic idea that underpins them all is, in some sense, to maximize utility, which is often defined in terms of well-being or related concepts. For instance, Jeremy Bentham, the founder of utilitarianism, described utility as the capacity of actions or objects to produce benefits, such as pleasure, happiness, and good, or to prevent harm, such as pain and unhappiness, to those affected.

Utilitarianism is a version of consequentialism, which states that the consequences of any action are the only standard of right and wrong. Unlike other forms of consequentialism, such as egoism and altruism, egalitarian utilitarianism considers either the interests of all humanity or all sentient beings equally. Proponents of utilitarianism have disagreed on a number of issues, such as whether actions should be chosen based on their likely results (act utilitarianism), or whether agents should conform to rules that maximize utility (rule utilitarianism). There is also disagreement as to whether total utility (total utilitarianism) or average utility (average utilitarianism) should be maximized.

The seeds of the theory can be found in the hedonists Aristippus and Epicurus who viewed happiness as the only good, the state consequentialism of the ancient Chinese philosopher Mozi who developed a theory to maximize benefit and minimize harm, and in the work of the medieval Indian philosopher Shantideva. The tradition of modern utilitarianism began with Jeremy Bentham, and continued with such philosophers as John Stuart Mill, Henry Sidgwick, R. M. Hare, and Peter Singer. The concept has been applied towards social welfare economics, questions of justice, the crisis of global poverty, the ethics of raising animals for food, and the importance of avoiding existential risks to humanity.

Benthamism, the utilitarian philosophy founded by Jeremy Bentham, was substantially modified by his successor John Stuart Mill, who popularized the term utilitarianism. In 1861, Mill noted that he had "reason for believing himself to be the first person who brought the word utilitarian into use", although he "did not invent it, but adopted it from a passing expression" in John Galt's 1821 novel Annals of the Parish. However, Mill seems to have been unaware that Bentham had used the term utilitarian in his 1781 letter to George Wilson and his 1802 letter to Étienne Dumont.

The importance of happiness as an end for humans has long been argued. Forms of hedonism were put forward by the ancient Greek philosophers Aristippus and Epicurus. Aristotle argued that eudaimonia is the highest human good. Augustine wrote that "all men agree in desiring the last end, which is happiness". The idea that conduct should to be judged by its consequences also existed within the ancient world. Consequentialist theories were first developed by the ancient Chinese philosopher Mozi, who proposed a system that sought to maximize benefit and eliminate harm. Mohist consequentialism advocated communitarian moral goods, including political stability, population growth, and wealth, but did not support the utilitarian notion of maximizing individual happiness.

Utilitarian ideas can also be found in the work of medieval philosophers. In medieval India, the 8th-century philosopher Śāntideva wrote that humanity ought "to stop all the present and future pain and suffering of all sentient beings, and to bring about all present and future pleasure and happiness." In medieval Europe, happiness was explored in depth by Thomas Aquinas in his Summa Theologica. During the Renaissance, one author whose work incorporated consequentialist ideas was the political philosopher Niccolò Machiavelli.

Utilitarianism as a distinct ethical position only emerged in the 18th century, and although it is usually thought to have begun with Jeremy Bentham, there were earlier writers who presented theories that were strikingly similar.^{[citation needed]}

Francis Hutcheson first introduced a key utilitarian phrase in An Inquiry into the Original of Our Ideas of Beauty and Virtue (1725): when choosing the most moral action, the amount of virtue in a particular action is proportionate to the number of people it brings happiness to. In the same way, moral evil, or vice, is proportionate to the number of people made to suffer. The best action is the one that procures the greatest happiness to the greatest numbers, and the worst is the one that causes the most misery. In the first three editions of the book, Hutcheson included various mathematical algorithms "to compute the Morality of any Actions." In doing so, he echoed the later-proposed hedonic calculus of Bentham.