Epicureanism is a system of philosophy founded in 307 BCE and based upon the teachings of Epicurus, an ancient Greek philosopher. Epicurus was an atomist and materialist, following in the steps of Democritus. His materialism led him to religious skepticism and a general attack on superstition and divine intervention. Epicureanism was originally a challenge to Platonism, and its main opponent later became Stoicism. It is a form of hedonism insofar as it declares pleasure to be its sole intrinsic goal. However, the concept that the absence of pain and fear constitutes the greatest pleasure, and its advocacy of a simple life, make it very different from hedonism as colloquially understood.

Following the Cyrenaic philosopher Aristippus, Epicurus believed that the greatest good was to seek modest, sustainable pleasure in the form of a state of ataraxia (tranquility and freedom from fear) and aponia (the absence of bodily pain) through knowledge of the workings of the world and limiting desires. Correspondingly, Epicurus and his followers generally withdrew from politics because it could lead to frustrations and ambitions that would conflict with their pursuit of virtue and peace of mind.

Few writings by Epicurus have survived. Diogenes Laertius has preserved three instructional letters attributed to Epicurus, as well as a list of the Principal Doctrines of Epicureanism. The letters to Herodotus and to Menoeceus are generally accepted as authentic works written by Epicurus himself. However, the letter addressed to Pythocles is often considered to be a compilation by one of his students, likely based on Epicurus’ original writings. There are also independent attestations of his ideas from his later disciples. The epic poem De rerum natura (Latin for "On the Nature of Things") by Lucretius presents the core arguments and theories of Epicureanism in one unified work. Many Epicurean texts have also been found on scrolls unearthed at the Villa of the Papyri in Herculaneum, mostly works written by the Epicurean philosopher Philodemus or his teacher Zeno of Sidon, along with fragments of works by Epicurus himself. Diogenes of Oenoanda, a wealthy Epicurean in the 2nd century CE, had a portico wall inscribed with tenets of the philosophy erected in Oenoanda, Lycia (present-day Turkey).

Epicureanism flourished in the Late Hellenistic period and during the Roman era, and many Epicurean communities were established in places such as Antioch, Alexandria, Rhodes, and Herculaneum. By the late 3rd century CE, Epicureanism had all but died out, being opposed by other philosophies (mainly Neoplatonism) that were then in the ascent. Interest in Epicureanism was resurrected in the Age of Enlightenment and continues in the modern era.

Epicurus, the founder of the Epicurean School, was born in 342/1 BCE on the island of Samos. In his early years, he studied under Pamphilus, a Platonist philosopher in Samos. Later, he received instruction from Nausiphanes of Teos, a follower of Democritus. Although Epicurus would later downplay these early influences, it is generally acknowledged that Nausiphanes had a significant impact on the development of his thought. At the age of eighteen, Epicurus traveled to Athens for military service. After completing his duties, he devoted himself entirely to philosophy while living in Colophon. Epicurus taught and gained followers in Mytilene, the capital of the island Lesbos, and then in Lampsacus. In Athens, Epicurus bought a property for his school called "Garden", which later became the name of Epicurus' school. 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. Community activities held some importance, particularly the observance of Eikas, a monthly social gathering. Some members were also vegetarians, as, from slight evidence, Epicurus did not eat meat, although no prohibition against eating meat was made.

The school's popularity grew and it became, along with Stoicism, Platonism, Peripateticism, and Pyrrhonism, one of the dominant schools of Hellenistic philosophy, lasting strongly through the later Roman Empire. Deciphered carbonized scrolls obtained from the library at the Villa of the Papyri in Herculaneum contain a large number of works by Philodemus, a late Hellenistic Epicurean, and Epicurus himself, attesting to the school's enduring popularity. Julius Caesar also leaned considerably toward Epicureanism, which led to his plea against the death sentence during the trial against Catiline, during the Catiline conspiracy where he spoke out against the Stoic Cato. His father-in-law, Lucius Calpurnius Piso Caesoninus, was also an adept of the school. In the 2nd century CE, comedian Lucian of Samosata and wealthy promoter of philosophy Diogenes of Oenoanda were prominent Epicureans.

After the death of Epicurus, leadership of the school passed to Hermarchus, who was originally from Mytilene. He was succeeded by Polystratus. Among Epicurus' closest students were Hermarchus, Polyaenus, and Metrodorus of Lampsacus.

One of the earliest Roman writers espousing Epicureanism was Amafinius, although his work has not survived. The Roman philosopher Cicero later wrote that he had listened to Phaedrus, who served as the head of the Athenian school around 90 BCE and had come to Rome. However, the most well-known follower of Epicureanism was the Roman poet Titus Lucretius Carus (c. 91–51 BCE). Lucretius expressed Epicurus' philosophical ideas in his didactic poem De Rerum Natura (On the Nature of Things). The main goal of this work was to free people from the fear of the gods and death, and to guide them toward achieving inner peace and tranquility. The Epicurean philosopher Philodemus of Gadara, until the 18th century only known as a poet of minor importance, rose to prominence as much of his work, along with other Epicurean material such as the lectures of Zeno of Sidon, was discovered in the Villa of the Papyri. Another ancient Epicurean is Diogenes of Oenoanda, in the 2nd century CE, who composed a large inscription at Oenoanda in Lycia. Diogenes Laërtius reports slanderous stories, circulated by Epicurus' opponents.