Utilitarianism is a moral theory based on the principle that "Actions are right in proportion as they tend to promote happiness, wrong as they tend to produce the reverse of happiness."
Mill sees happiness as humans' primary desire; hence, it is the basis for morality and justice. In defense of utilitarianism, Mill defines and tries to validate his position by relating justice and utility and arguing that happiness is the foundation of justice. While the whole argument can be presented in many ways, Mill tried to vindicate Utilitarianism from accusations such as: not protecting individual rights, measuring every thing in life with the same standard, and over simplifying happiness.

From the man who once said: "Whatever crushes individuality is despotism, by whatever name it may be called", it seems that Mill ignored the absence of individuals' appreciation in utilitarianism. Knowing that Mill was raised in a strict utilitarian environment, and the mental struggle he went through to validate his theory is key to understanding this work. Instead of judging this work as a reasonable philosophical work, the reader should approach it as Mill's introspection through which he tried to justify his belief in utilitarianism.

The importance of J.S. Mill's Utilitarianism as a statement of the fundamental tenets of his school is indisputable. Equally unquestionable is the great influence utilitarianism has exerted upon the development of Anglo-American moral philosophy. These facts underscore the necessity of carefully considering whether utilitarianism, as articulated by Mill, offers a sensible or persuasive account of morality.

In utilitarianism, "utility" is synonymous with "happiness"; both denote "pleasure itself, together with exemption from pain." Hence utilitarianism is also referred to as the "greatest happiness principle." However, the latter slogan is misleading insofar as "greatest" is taken to refer merely to quantity. Mill holds that pleasures can be compared not only quantitatively but also qualitatively; "some kinds of pleasure are more desirable and more valuable than others." There is an hierarchy of pleasures, and the happy life will be the life that contains the "greatest" pleasures both in the sense of the "best" or "highest" as well as the "most."

Indeed, Mill argues that the higher pleasures are such that no one who has experienced them would be willing to trade them in for "any quantity" of lower pleasures. "A being of higher faculties requires more to make him happy, is capable probably of more acute suffering, and certainly accessible to it at more points," Mill says, "but in spite of these liabilities, he can never really wish to sink into what he feels to be a lower grade of existence." Note that this is an empirical argument which implicitly postulates that the pain and suffering caused by heightened sensitivities is outweighed by the pleasures brought on by the higher faculties. At the back of this postulate is the idea, not articulated by Mill but informing his thought, that the supreme pleasure is the pleasure associated with knowing (cf. Mill's statement that "[n]ext to selfishness, the principal cause which makes life unsatisfactory is want of mental cultivation"). Philosophers prior to Mill had taught that the different capacities for enjoyment derive from different capacities for knowing. Partly on that basis, those philosophers concluded that philosophy was the best (happiest) way of life for man.

But not everyone exhibits a burning desire to be a philosopher, to put it mildly. This observation points to a fallacy in Mill's argument. From the fact (assuming it to be a fact) that no one could "really wish to sink into what he feels to be a lower grade of existence," it does not follow that no one could prefer a lower grade of existence to a higher grade of existence. Even if no one wishes to fall to a lower level of existence, it does not follow that everyone wishes to rise to a higher level of existence. For the higher faculties are not developed "by nature," but "by art," ie, through human effort, and most people, as Nietzsche observed, are lazy. What most people seem to want is to retain the "grade of existence" they currently enjoy, but in better conditions (better house, better car, better spouse, bigger bank account, etc.). It will not do to advert to Mill's quip about the pig and the human being and the fool and Socrates as a rejoinder. For in Chapter IV Mill explicitly holds that there are different conceptions/objects of happiness, and it is all too clear that he has no way of rank ordering those different conceptions. That account implicitly undermines the hierarchy of pleasures that he set out to establish in Chapter II. Indeed, the three objects of happiness he lists in Chapter IV (money, power and fame) would seem to be in considerable tension with utilitarian ethics altogether.

That tension has its roots in Mill's incoherent attempt to derive ethics from psychology. This is a Humean criticism. Mill says that all men desire their own happiness. Happiness being the end of human action, it is also the standard of morality, which regulates human action for the benefit of all mankind. Hence the utilitarian standard, qua moral standard, "is not the agent's own greatest happiness, but the greatest amount [sic!] of happiness altogether." But Mill does not explain how he gets from men desiring their own happiness to men acting to make other men happy. He fails to explain how a concern with being ethical can be located within his account of human beings as desiring happiness conceived as pleasure. That is, how does being concerned with living a pleasant life for myself generate a concern for acting morally? Mill attempts to ground morality in the sentiments of sympathy and sociality, but there are severe limits on the sort of ethics that can be generated by those sentiments. What this means is that Mill's doctrine fails to explain why we would take his doctrine seriously. Bluntly stated, the fact that we take Mill's ethical writings seriously is already a proof of the false foundations of Mill's ethics.

If someone would like to know what Utilitarianism is, this is the book.
But if someone thinks to find in the Utilitarianism a moral standard to follow, this is just one of the books.
According to the Mill' theory, we should always act in a manner that will maximize overal happines and in this essay John Stuart Mill wrote which are the effects of each possible action we may perform.
The Speech on Capital Punishment tells one of this possible action.

The foundation of consequentialist ethical theory, John Stuart Mill's Utilitarianism is a must read for anybody who wants to understand ethical theory. While we may debate what makes action right or wrong, Mill's take is one that must be acknowledged.

One of the Classical School economists explains and defends a system of ethics that counted among its adherents Ludwig von Mises, one of the great Austrian School economists and philosophers.
Utilitarianism, in John Stuart Mill's day and our own, periodically comes under attack from the spokesmen of organized religion. But Mill holds that his philosophy is completely compatible with religious morals. Mill even writes that the founder of Christianity was a utilitarian. Makes sense when we realize that one of the main features of the early Christians was jettisoning Judaism commandments that seem to have no obvious utility (usefulness). That attitude lead them to eventually discard the entire Torah.
Mill imbibed Utilitarianism from his father -- British East India Co. executive and writer James Mill -- and their friend Jeremy Bentham. The two tablets of Utilitarianism are pleasure (acquisition of) and pain (avoidance of). Reduced to one it is the "greatest happiness principle." Mill argues persuasively that these things are more hard-wired into humans than almost everything else. The pursuit of virtue, which some in organized religion see as being at odds with Utilitarianism, is actually a form of the pursuit of happiness for the virtue-seeker, those around him/her, and/or future generations. This adds to the "public good," which is at the peak of Mill's values pyramid.
Utilitarian concepts are all over America's founding documents, especially the Constitution. Interestingly, and ironically, Mill's essay was published at the time of the Constitution's greatest crisis -- the Civil War (1863). Mill makes no mention of the crisis or America's earlier successful marriage of Utilitarianism and federalism/limited government.
Mill's "public good" and the U.S. Constitution's "general welfare" clauses helped open the gates to big government, Ayn Rand and other individual rights advocates point out. Sad but true. Although his ideas contain seeds for the modern welfare state, Mill meant his public good to be best achieved by free-acting individuals getting little or no prompting from government.
How does the individualized commandment of "love thy neighbor as thyself" get turned into the collectivist Social Security Administration? Perhaps the psychiatric profession can explain it. I can't.