Epicurus was an Hellenistic Greek philosopher, an atomic materialist, following in the steps of Democritus. His materialism led him to a general attack on superstition and divine intervention. Following Aristippus — about whom we know very little — Epicurus believed that the greatest good was to seek pleasure. This doctrine is also known as hedonism.
For Epicurus, pleasure was obtained by knowledge (freedom from fear), friendship, and living a virtuous and temperate life. Epicurus did not articulate a broad system of social ethics that has survived.
Epicurean materialism is presented very simply, but anticipates a great deal of later scientific discovery in important respects. Dalton's atomic theory and Darwin's theory of evolution can both be seen in Epicurean writings.
Some writings by Epicurus have survived. In addition, many scholars consider the epic poem On the Nature of Things by Lucretius to present in one unified work the core arguments and theories in Epicurus's writings.
Epicureanism emphasizes the neutrality of gods on earth and that they do not interfere with the world we live in. It also states that gods, matter and souls are all made from the same thing (atoms). Souls are made from atoms, and gods possess souls, but their souls adhere to the bodies without escaping. In the case of humans we do have the same kind of souls, but the forces between our atoms do not possess the fortitude to hold the soul forever.
Epicureanism is probably the first philosophical school which introduced the social contract, in that the laws established by this school of thought are based on mutual agreement, not divine decree.
Epicurus' school, called "The Garden," seems to have been a moderately ascetic community which rejected the political limelight of Athenian philosophy. They were fairly cosmopolitan by Athenian standards, including women and slaves, and were probably vegetarians.
Epicureanism was the main opponent of Stoicism. Epicurus and his followers shunned politics and as such it was never a major philosophy. After the death of Epicurus, its main proponent was the Roman Lucretius. It had all but died out by the end of the Roman Empire, and was again resurrected by the atomist Pierre Gassendi during the Enlightenment.