Delphi is both an archaeological site and a modern town in Greece on the south-western spur of Mount Parnassus in the valley of Phocis. Delphi was the site of the Delphic oracle, the most important oracle in the classical Greek world, when it was a major site for the worship of the god Apollo after he slew the Python, a deity who lived there and protected the navel of the Earth. The Homeric Hymn to Delphic Apollo recalled that the ancient name of this site had been Krisa. His sacred precinct in Delphi was a panhellenic sanctuary, where every four years athletes from all over the Greek world competed in the Pythian Games, one of the four panhellenic (or stephanitic) games, precursors to the Modern Olympics.

Delphi was revered throughout the Greek world as the site of the omphalos stone, the centre of the earth and the universe. In the inner hestia ("hearth") of the Temple of Apollo, an eternal flame burned. After the battle of Plataea, the Greek cities extinguished their fires and brought new fire from the hearth of Greece, at Delphi; in the foundation stories of several Greek colonies, the founding colonists were first dedicated at Delphi.

Delphi site is located in lower central Greece, on multiple plateau/terraces along the slope of Mount Parnassus, and includes the Sanctuary of Apollo, the site of the ancient Oracle. This semicircular spur is known as Phaedriades, and overlooks the Pleistos Valley. Southwest of Delphi, about 15 km (9.3 mi) away, is the harbor-city of Kirrha on the Corinthian Gulf.

The name Delphois comes from the same root as δελφύς delphus, "womb" and may indicate archaic veneration of Gaia, Grandmother Earth, and the Earth Goddess at the site. Apollo is connected with the site by his epithet Δελφίνιος Delphinios, "the Delphinian." The epithet is connected with dolphins in the Homeric Hymn to Apollo (line 400), recounting the legend of how Apollo first came to Delphi in the shape of a dolphin, carrying Cretan priests on his back. The Homeric name of the oracle is Pytho (Πυθώ).

Another legend held that Apollo walked to Delphi from the north and stopped at Tempe, a city in Thessaly to pick laurel, a plant sacred to him (generally known in English as the bay tree). In commemoration of this legend, the winners at the Pythian Games received a wreath of laurel (bay leaves) picked in the Temple.

Delphi became the site of a major temple to Phoebus Apollo, as well as the Pythian Games and the famous prehistoric oracle. Even in Roman times, hundreds of votive statues remained, described by Pliny the Younger and seen by Pausanias. Supposedly carved into the temple were three phrases: γνωθι σεαυτόν (gnōthi seautón = "know thyself") and μηδέν άγαν (mēdén ágan = "nothing in excess"), and Εγγύα πάρα δ'ατη (engýa pára d'atē = "make a pledge and mischief is nigh"), as well as a large letter E. Among other things epsilon signifies the number 5. Plutarch's essay on the meaning of the “E at Delphi" is the only literary source for the inscription. In ancient times, the origin of these phrases was attributed to one or more of the Seven Sages of Greece, though ancient as well as modern scholars have doubted the legitimacy of such ascriptions. According to one pair of scholars, "The actual authorship of the three maxims set up on the Delphian temple may be left uncertain. Most likely they were popular proverbs, which tended later to be attributed to particular sages."

From a late myth that deviates from much older ones, when young, Apollo killed the chthonic serpent Python, named Pythia in older myths, but according to some later accounts his wife, Pythia, who lived beside the Castalian Spring, according to some because Python had attempted to rape Leto while she was pregnant with Apollo and Artemis. The bodies of the pair were draped around his Rod, which, with the wings created the caduceus symbolic of the god. This spring flowed toward the temple but disappeared beneath, creating a cleft which emitted vapors that caused the Oracle at Delphi to give her prophecies. Apollo killed Python but had to be punished for it, since she was a child of Gaia. The shrine dedicated to Apollo was originally dedicated to Gaia and then, possibly to Poseidon. The name Pythia remained as the title of the Delphic Oracle. As punishment for this murder Apollo was sent to serve in menial tasks for eight years. A festival, the Septeria, was performed annually portraying the slaying of the serpent, the flight, the atonement and the return of the God. The Pythian Games took place every four years to commemorate his victory. Another regular Delphi festival was the "Theophania" (Θεοφάνεια), an annual festival in spring celebrating the return of Apollo from his winter quarters in Hyperborea. The culmination of the festival was a display of an image of the gods, usually hidden in the sanctuary, to worshippers. The "Theoxenia" was held each summer, centred on a feast for "gods and ambassadors from other states".

Erwin Rohde wrote that the Python was an earth spirit, who was conquered by Apollo, and buried under the Omphalos, and that it is a case of one deity setting up a temple on the grave of another. Another view holds that Apollo was a fairly recent addition to the Greek pantheon coming originally from Lydia. The Etruscans coming from northern Anatolia also worshiped Apollo, and it may be that he was originally identical with Mesopotamian Aplu, an Akkadian title meaning "son", originally given to the plague God Nergal, son of Enlil. Apollo Smintheus (Greek Απόλλων Σμινθεύς), the mouse killer eliminates mice, a primary cause of disease, hence he promotes preventive medicine.

Delphi is perhaps best-known for the oracle at the sanctuary that became dedicated to Apollo during the classical period. According to Aeschylus in the prologue of the Eumenides, it had origins in prehistoric times and the worship of Gaia. In the last quarter of the 8th century BC there is a steady increase of artifacts found at the settlement site in Delphi, which was a new, post-Mycenaean settlement of the late 9th century. Pottery and bronze work as well as tripod dedications continue in a steady stream, in comparison to Olympia. Neither the range of objects nor the presence of prestigious dedications proves that Delphi was a focus of attention for worshipers of a wide range, but the strong representation of high value goods, found in no other mainland sanctuary, certainly encourages that view.

The priestess of the oracle at Delphi was known as the Pythia. Apollo spoke through his oracle, who had to be an older woman of blameless life chosen from among the peasants of the area. The sibyl or prophetess took the name Pythia and sat on a tripod seat over an opening in the earth. When Apollo slew Python, its body fell into this fissure, according to legend, and fumes arose from its decomposing body. Intoxicated by the vapors, the sibyl would fall into a trance, allowing Apollo to possess her spirit. In this state she prophesied. It has been postulated that a gas high in ethylene, known to produce violent trances, came out of this opening, though this theory remains debatable. While in a trance the Pythia "raved" - probably a form of ecstatic speech - and her ravings were "translated" by the priests of the temple into elegant hexameters. People consulted the Delphic oracle on everything from important matters of public policy to personal affairs. The oracle could not be consulted during the winter months, for this was traditionally the time when Apollo would live among the Hyperboreans. Dionysus would inhabit the temple during his absence.

H.W. Parke writes that the foundation of Delphi and its oracle took place before the times of recorded history and its origins are obscure, but dating to the worship of the Great Goddess, Gaia.

The Oracle exerted considerable influence throughout the Greek world, and she was consulted before all major undertakings: wars, the founding of colonies, and so forth. She also was respected by the semi-Hellenic countries around the Greek world, such as Lydia, Caria, and even Egypt.

The Oracle benefited by the Macedonian Kings. Later it was placed under the protection of the Aetolians. After a brief period the Romans started to emerge, who even protected it from a dangerous barbarian invasion in 109 B.C. and 105 B.C. A major reorganization was initiated which was interrupted by the wars of Mithridatic Wars and Sulla who took many rich offerings from the Oracle. Barbarian invasions were the direct cause of harm, burning the Temple, which had been damaged severely by an earthquake in 83 B.C. The result was that the Oracle fell in decay and the surrounding area became impoverished. The small population led to difficulties in finding the right people to fill the posts required. Its credibility waned due to doubtful predictions. When Nero came to Greece in 66 AD, he took away over 500 of the best statues from Delphi to Rome. Subsequent Roman emperors from the Flavian dynasty contributed significantly towards its restoration. Hadrian offered complete autonomy. Also Plutarch was a significant factor by his presence as a chief priest. However, barbarian raids during the reign of Marcus Aurelius and removal of statues and other riches (practically looting) by Constantine the Great caused it to decay. The short reign of Julian the Apostate was not capable to improve its situation. Despite this, the Oracle existed until it was ordered to close by emperor Theodosius I in 395 AD. The site was abandoned for almost 100 years, until Christians started to settle permanently in the area and established a small town about 600 AD by the name Kastri.

The Delphic Sibyl was a legendary prophetic figure who was said to have given prophecies at Delphi shortly after the Trojan War. The prophecies attributed to her circulated in written collections of prophetic sayings, along with the oracles of figures such as Bakis. The Sibyl had no connection to the oracle of Apollo, and should not be confused with the Pythia.

Modern Delphi is situated immediately west of the archaeological site and hence is a popular tourist destination. It is on a major highway linking Amfissa along with Itea and Arachova. There are many hotels and guest houses in the town, and many taverns and bars. The main streets are narrow, and often one-way. Delphi also has a school, a lyceum, a church and a square (plateia). The Trans European Footpath E4 passes through the east end of the town. In addition to the archaeological interest, Delphi attracts tourists visiting the Parnassus Ski Center and the popular coastal towns of the region. The town has a population of 2,373 people while the population of the municipality of Delphi, including Chrisso (ancient Krissa), is 3,511.

In medieval times Delphi was also called Kastri and was built on the archaeological site. The residents had used the marble columns and structures as support beams and roofs for their improvised houses, a usual way of rebuilding towns that were partially or totally destroyed, especially after the earthquake in 1580, which demolished several towns in Phocis. In 1893 archaeologists from the École française d'Athènes finally located the actual site of ancient Delphi and the village was moved to a new location, west of the site of the temples.

The Delphi Archaeological Museum is at the foot of the main archaeological complex, on the east side of the village, and on the north side of the main road. The museum houses an impressive collection associated with ancient Delphi, including the earliest known notation of a melody, the famous Charioteer, golden treasures discovered beneath the Sacred Way, and fragments of reliefs from the Siphnian Treasury. Immediately adjacent to the exit (and overlooked by most tour guides) is the inscription that mentions the Roman proconsul Gallio.

Entries to the museum and to the main complex are separate and chargeable, and a reduced rate ticket gets entry to both. There is a small cafe, and a post office by the museum. Slightly further east, on the south side of the main road, is the Gymnasium and the Tholos. Entry to these is free.