For Epicurus, the highest pleasure (tranquility and freedom from fear) was obtained by knowledge, friendship, and living a virtuous and temperate life. He lauded the enjoyment of simple pleasures, by which he meant abstaining from bodily desires, such as sex and appetites, verging on asceticism. He argued that when eating, one should not eat too richly, for it could lead to dissatisfaction later, such as the grim realization that one could not afford such delicacies in the future. Likewise, sex could lead to increased lust and dissatisfaction with the sexual partner. Epicurus did not articulate a broad system of social ethics that has survived.

Some writings by Epicurus have survived. Some scholars consider the epic poem On the Nature of Things by Lucretius to present in one unified work the core arguments and theories of Epicureanism. Many of the papyrus scrolls unearthed at the Villa of the Papyri at Herculaneum are Epicurean texts. At least some are thought to have belonged to the Epicurean Philodemus.

The school of Epicurus, called "The Garden," was based in Epicurus' home and garden. It had a small but devoted following in his lifetime. Its members included Hermarchus, Idomeneus, Colotes, Polyaenus, and Metrodorus. Epicurus emphasized friendship as an important ingredient of happiness, and the school seems to have been a moderately ascetic community that rejected the political limelight of Athenian philosophy. They were fairly cosmopolitan by Athenian standards, including women and slaves, and were probably vegetarians (Stevenson 2005).

A library, dubbed the Villa of the Papyri, in Herculaneum, owned by Julius Caesar's father-in-law, Lucius Calpurnius Piso Caesoninus, was preserved by the eruption of Vesuvius in 79 CE, and was found to contain a large number of works by Philodemus, a late Hellenistic Epicurean, and Epicurus himself, attesting to the school's enduring popularity. The task of unrolling and deciphering the charred papyrus scrolls continues today.

After the official approval of Christianity by Constantine, Epicureanism was repressed. Epicurus' materialist theories that the gods were physical beings composed of atoms who were unconcerned with human affairs and had not created the universe, and his general teaching that one's own pleasure, rather than service to God, was the greatest good were essentially irreconcilable with Christian teachings. The school endured a long period of obscurity and decline.

The early Christian writer Lactantius criticizes Epicurus at several points throughout his Divine Institutes. In Dante's Divine Comedy, the Epicureans are depicted as heretics suffering in the sixth circle of hell. The word for a heretic in the Talmudic literature is "Apiqoros" and Epicurus is titled in Modern Greek idiom as the "Dark Philosopher".

By the 16th century, the works of Lucretius and Diogenes Laertius were being printed in Europe. In the 17th century, the French Franciscan priest, scientist and philosopher Pierre Gassendi wrote two books forcefully reviving Epicureanism. Shortly thereafter, and clearly influenced by Gassendi, Walter Charleton published several works on Epicureanism in English. Attacks by Christians continued, most forcefully by the Cambridge Platonists.

In the following times, there was a resurgence of Epicurean philosophy: in the Modern Age, scientists adopted atomist theories, while materialist philosophers embraced Epicurus' hedonist ethics and restated his objections to natural teleology.

Epicureanism emphasizes the neutrality of the gods, that they do not interfere with human lives. It states that gods, matter, and souls are all comprised of atoms. Souls are made from atoms, and gods possess souls, but their souls adhere to their bodies without escaping. Humans have the same kind of souls, but the forces binding human atoms together do not hold the soul forever. The Epicureans also used the atomist theories of Democritus and Leucippus to assert that man has free will. They held that all thoughts are merely atoms swerving randomly. This explanation served to satisfy people who wondered anxiously about their role in the universe.

The Riddle of Epicurus or Epicurean paradox is the earliest known description of the Problem of evil and is a famous argument against the existence of an all-powerful and providential God or gods. As translated by David Hume in the Dialogues concerning Natural Religion:

If God is willing to prevent evil but is not able to Then He is not omnipotent.

If He is able, but not willing Then He is malevolent.

If He is both able and willing Then whence cometh evil?

If He is neither able nor willing Then why call Him God?

Epicurus did not, however, deny the existence of gods. Fully aware of the fate of Socrates when brought up on a charge of impiety, Epicurus avoided expressing overt atheism. Rather, he conceived the gods as blissful and immortal yet material beings made of atoms inhabiting the metakosmia: empty spaces between worlds in the vastness of infinite space. In spite of his nominal recognition of the gods, the practical effect of this materialistic explanation of the gods' existence and their complete non-intervention in human affairs renders his philosophy atheistic on a practical level but avoids the charge of atheism on the theoretical level.

In Dante's Divine Comedy, the flaming tombs of the Epicureans are located within the sixth circle of hell (Inferno, Canto X). Similarly, according to Jewish Mishnah, Epicureans (apiqorsim, people who share the beliefs of the movement) are among the people who do not have a share of the "World-to-Come" (afterlife or the world of the Messianic Era).

Parallels may be drawn to Buddhism, which similarly emphasizes a lack of divine interference and aspects of its atomism. Buddhism also resembles Epicureanism in its temperateness, including the belief that great excesses lead to great dissatisfaction.

The philosophy originated by Epicurus flourished for seven centuries. It propounded an ethic of individual pleasure as the sole or chief good in life. Hence, Epicurus advocated living in such a way as to derive the greatest amount of pleasure possible during one’s lifetime, yet doing so moderately in order to avoid the suffering incurred by overindulgence in such pleasure. The emphasis was placed on pleasures of the mind rather than on physical pleasures. Therefore, according to Epicurus, with whom a person eats is of greater importance than what is eaten. Unnecessary and, especially, artificially produced desires were to be suppressed. Since learning, culture, and civilization as well as social and political involvements could give rise to desires that are difficult to satisfy and thus result in disturbing one’s peace of mind, they were discouraged. Knowledge was sought only to rid oneself of religious fears and superstitions, the two primary fears to be eliminated being fear of the gods and of death. Viewing marriage and what attends it as a threat to one’s peace of mind, Epicurus lived a celibate life but did not impose this restriction on his followers.

The philosophy was characterized by an absence of divine principle. Lawbreaking was counseled against because of both the shame associated with detection and the punishment it might bring. Living in fear of being found out or punished would take away from pleasure, and this made even secret wrongdoing inadvisable. To the Epicureans, virtue in itself had no value and was beneficial only when it served as a means to gain happiness. Reciprocity was recommended, not because it was divinely ordered or innately noble, but because it was personally beneficial. Friendships rested on the same mutual basis, that is, the pleasure resulting to the possessors. Epicurus laid great emphasis on developing friendships as the basis of a satisfying life.

Of all the things which wisdom has contrived which contribute to a blessed life, none is more important, more fruitful, than friendship

While the pursuit of pleasure formed the focal point of the philosophy, this was largely directed to the "static pleasures" of minimizing pain, anxiety, and suffering. In fact, Epicurus referred to life as a “bitter gift”.

When we say...that pleasure is the end and aim, we do not mean the pleasures of the prodigal or the pleasures of sensuality, as we are understood to do by some through ignorance, prejudice, or willful misrepresentation. By pleasure, we mean the absence of pain in the body and of trouble in the soul. It is not by an unbroken succession of drinking bouts and of revelry, not by sexual lust, nor the enjoyment of fish and other delicacies of a luxurious table, which produce a pleasant life; it is sober reasoning, searching out the grounds of every choice and avoidance, and banishing those beliefs through which the greatest tumults take possession of the soul.

The Epicureans believed in the existence of the gods but believed that the gods were made of atoms just like everything else. It was thought that the gods were too far away from the earth to have any interest in what man was doing; so it did not do any good to pray or to sacrifice to them. The gods, they believed, did not create the universe, nor did they inflict punishment or bestow blessings on anyone, but they were supremely happy; this was the goal to strive for during one’s own human life.

Epicureanism rejects immortality and mysticism; it believes in the soul, but suggests that the soul is as mortal as the body. Epicurus rejected any possibility of an afterlife, while still contending that one need not fear death: "Death is nothing to us; for that which is dissolved, is without sensation, and that which lacks sensation is nothing to us."Bertrand Russell critiques Epicureanism's conception of death:

The fear of death is so deeply rooted in instinct that the gospel of Epicurus could not, at any time, make a wide popular appeal; it remained always the creed of a cultivated minority. Even among philosophers, after the time of Augustus, it was, as a rule, rejected in favor of Stoicism. It survived, it is true, though with diminishing vigor, for six hundred years after the death of Epicurus; but as men became increasingly oppressed by the miseries of our terrestrial existence, they demanded continually stronger medicine from philosophy or religion. The philosophers took refuge, with few exceptions, in Neoplatonism; the uneducated turned to various Eastern superstitions, and then, in continually increasing numbers, to Christianity, which, in its early form, placed all good in life beyond the grave, thus offering men a gospel which was the exact opposite of that of Epicurus.

Epicurus was an early thinker to develop the notion of justice as a social contract. He defined justice as an agreement "neither to harm nor be harmed." The point of living in a society with laws and punishments is to be protected from harm so that one is free to pursue happiness. Because of this, laws that do not contribute to promoting human happiness are not just. He gave his own unique version of the Ethic of Reciprocity, which differs from other formulations by emphasizing minimizing harm and maximizing happiness for oneself and others:

It is impossible to live a pleasant life without living wisely and well and justly (agreeing 'neither to harm nor be harmed'). And it is impossible to live wisely and well and justly without living a pleasant life.

Epicureanism incorporated a relatively full account of the social contract theory, following after a vague description of such a society in Plato's Republic. The social contract theory established by Epicureanism is based on mutual agreement, not divine decree.