There is much contradictory information regarding the parentage of the Dioscuri. In the Homeric Odyssey they are the sons of Tyndareus alone, but they were sons of Zeus in the Hesiodic Catalogue of Women. The conventional account, attested first in Pindar's Nemean Odes, combined these paternities so that only Pollux was fathered by Zeus, while Leda and her husband Tyndareus conceived Castor. This explains why they were granted an alternative immortality. The figure of Tyndareus may have entered their tradition to explain their archaic name Tindaridai in Spartan inscriptions, in turn occasioning incompatible accounts of their parentage. Their other sisters were Timandra, Phoebe, and Philonoe.

Castor and Pollux are sometimes both mortal, sometimes both divine. One consistent point is that if only one of them is immortal, it is Pollux. In Homer's Iliad, Helen looks down from the walls of Troy and wonders why she does not see her brothers among the Achaeans; the narrator remarks that they are both already dead and buried back in their homeland of Lacedaemon, suggesting that in some early traditions both were mortal. Their death and the shared immortality offered by Zeus was the material of the lost Cypria in the Epic Cycle.

The Dioscuri were regarded as helpers of mankind and held to be patrons of travellers and of sailors in particular, who invoked them to seek favourable winds. Their role as horsemen and boxers also led to them being regarded as the patrons of athletes and athletic contests. They characteristically intervened at the moment of crisis, aiding those who honoured or trusted them.

Ancient Greek authors tell a number of versions of the story of Castor and Pollux. Homer portrays them initially as ordinary mortals, treating them as dead in the Iliad, where Helen names "Castor the horse breaker and the boxer Polydeuces, my brothers." But in the Odyssey they are described as both alive, even though "the grain-bearing earth holds them": they have "honour equal to gods," living on alternate days because of the intervention of Zeus.

In both the Odyssey and in Hesiod they are described as the sons of Tyndareus and Leda, whereas in Pindar Pollux is the son of Zeus while Castor is the son of the mortal Tyndareus. The theme of ambiguous parentage is not unique to them; similar characterisations appear in the stories of Herakles and Theseus. The Dioscuri are also invoked in Alcaeus' fragment 34a, and they appear together in two plays by Euripides, Helen and Electra.

Cicero tells how Simonides of Ceos was rebuked by his patron Scopas for devoting too much space to praising Castor and Pollux in an ode celebrating Scopas' chariot victory. Shortly afterwards Simonides was told that two young men wished to speak with him; after he had left the banqueting room, the roof fell in and crushed Scopas and his guests. According to the ancient sources the horse of Castor was named Cyllarus.

As Argonauts: Both Dioscuri were excellent horsemen and hunters who participated in the hunt for the Calydonian Boar and later joined the crew of Jason's ship, the Argo. During the expedition, Pollux took part in a boxing contest and defeated King Amycus of the Bebryces, a savage people of Bithynia. After returning from the voyage, the Dioscuri helped Jason and Peleus to destroy the city of Iolcus in revenge for the treachery of its king Pelias.

Rescuing Helen: When their sister Helen was abducted by Theseus, the half-brothers invaded his kingdom of Attica to rescue her. In revenge they abducted Theseus's mother Aethra, took her to Sparta, and set his rival Menestheus on the throne of Athens. Aethra was forced to become Helen's slave and was ultimately returned home by her grandsons Demophon and Acamas after the fall of Troy.

The Leucippides and the feud: Castor and Pollux aspired to marry the Leucippides ("daughters of the white horse"), Phoebe and Hilaeira, daughters of Leucippus. Both women were already betrothed to the twins' cousins Lynceus and Idas of Messenia, sons of Tyndareus's brother Aphareus. Castor and Pollux carried the women off to Sparta, where Phoebe bore Mnesileos to Pollux and Hilaeira bore Anogon to Castor, beginning a family feud among the four cousins.

The cousins had earlier carried out a cattle-raid in Arcadia together but fell out over the division of the meat: the gigantic Idas tricked Castor and Pollux out of the whole herd by proposing that it be split between whichever pair finished eating first, then devoured both his and his brother's portions. The duped twins vowed revenge.

Later, when Idas and Lynceus were visiting Sparta, Castor and Pollux set out to steal their cousins' herd. Castor climbed a tree to keep watch while Pollux freed the cattle, but the keen-sighted Lynceus spotted Castor; Idas ambushed and fatally wounded him with a spear, though not before Castor warned Pollux. In the ensuing brawl Pollux killed Lynceus, and as Idas was about to kill Pollux, Zeus, watching from Olympus, hurled a thunderbolt that killed Idas and saved his son.

Returning to the dying Castor, Pollux was offered by Zeus the choice of spending all his time on Mount Olympus or giving half his immortality to his mortal brother. He chose the latter, enabling the twins to alternate between Olympus and Hades. The brothers became the two brightest stars in the constellation Gemini: Castor (Alpha Geminorum) and Pollux (Beta Geminorum). As emblems of immortality and death, the Dioscuri, like Heracles, were said to have been initiated into the Eleusinian Mysteries, and in some myths Poseidon rewarded them with horses and the power to aid shipwrecked men.

Castor and Pollux are consistently associated with horses in art and literature and are widely depicted as helmeted horsemen carrying spears. On votive reliefs they appear with symbols of twinhood, such as the dokana (two upright beams joined by cross-beams), a pair of amphorae, a pair of shields, or a pair of snakes. They are often shown wearing felt caps, sometimes with stars above.

They are depicted on metopes from Delphi showing them on the voyage of the Argo and rustling cattle with Idas. Greek vases regularly show them capturing Phoebe and Hilaeira, as Argonauts, in religious ceremonies, and at the delivery to Leda of the egg containing Helen. They can be recognized in some vase-paintings by the skull-cap they wear, the pilos, which was already explained in antiquity as the remnants of the egg from which they hatched. Dares Phrygius described them as "blond haired, large eyed, fair complexioned, and well-built with trim bodies."

The Dioskouroi were worshipped by the Greeks and Romans alike; there were temples to the twins in Athens, such as the Anakeion, and in Rome, as well as shrines in many other locations of the ancient world. They grew up in Sparta in the royal household of Tyndareus and were particularly important to the Spartans, who associated them with the tradition of dual kingship. A uniquely Spartan aniconic representation of the Tyndaridai was two upright posts joined by a cross-bar; as protectors of the army this "beam figure" or dokana was carried before the troops on campaign, and when the army marched to war one king remained at home, accompanied by one of the Twins.

Their heroon or grave-shrine was on a mountain top at Therapne across the Eurotas from Sparta, at a shrine known as the Menelaeion, where Helen, Menelaus, Castor and Pollux were all said to be buried. Castor was also venerated in the region of Kastoria in northern Greece. They were commemorated both as gods on Olympus worthy of holocaust and as deceased mortals in Hades whose spirits had to be propitiated by libations. The pear tree was sacred to them at Sparta, and the standard Spartan oath was to swear "by the two gods."

The rite of theoxenia ("god-entertaining") was particularly associated with Castor and Pollux: the two deities were summoned to a table laid with food, in private homes or in public hearths, and are sometimes shown arriving at a gallop over a food-laden table. The domestic setting of the theoxenia was a characteristic distinction accorded to the Dioskouroi. The festival of Anakeia was held at Athens in their honor, where they also bore the name Anakes ("Lords"); Pausanias records that the people of Amphissa celebrated mystery rites for the so-called Anaktes Boys, variously identified with the Dioscuri, the Curetes, or the Cabeiri.

From the 5th century BCE the brothers were revered by the Romans, probably through cultural transmission via the Greek colonies of Magna Graecia in southern Italy. An archaic Latin inscription found at Lavinium reading Castorei Podlouqueique qurois ("To Castor and Pollux, the Dioskouroi") suggests direct transmission from the Greeks, the word "Podlouquei" being effectively a transliteration of the Greek Polydeukes.

The Temple of Castor and Pollux in the Roman Forum was built to fulfill a vow made by Aulus Postumius Albus Regillensis in gratitude for the Roman victory at the Battle of Lake Regillus in 495 BCE; according to legend the twins fought at the head of the army and brought news of the victory back to Rome. The Romans believed the twins aided them on the battlefield, and their role as horsemen made them especially attractive to the Roman equites: each year on 15 July, the Feast Day of the Dioskouroi, 1,800 equestrians paraded through Rome in full military attire. The twins are also represented in the Circus Maximus by the use of eggs as lap counters, and Photius wrote that Polydeuces was a lover of Hermes, who gave him the Thessalian horse Dotor.

Christianization: Even after the rise of Christianity the Dioskouroi continued to be venerated; the 5th-century pope Gelasius I attested to a "cult of Castores" the people did not want to abandon. In some instances the twins were absorbed into a Christian framework, depicted alongside the Twelve Apostles or with Saint Peter. The church sought to replace them with equivalent pairs: Saints Peter and Paul as patrons of travelers, and Saints Cosmas and Damian as healers. The scholar Dennis MacDonald identifies Castor and Pollux as models for the brothers James and John, the "Sons of Thunder," in the Gospel of Mark, and the Acts of the Apostles mentions the Dioskouroi as the figurehead of an Alexandrian ship boarded by Paul (Acts 28:11).

The heavenly twins appear throughout Indo-European tradition as the effulgent brother-horsemen: the Vedic Ashvins, the Lithuanian Ašvieniai, and possibly the Germanic Alcis, reflecting the twins' origin as the Indo-European horse twins.

The Etruscans venerated the pair as Kastur and Pultuce, collectively the tinas cliniiaras ("Sons of Tinia," the Etruscan counterpart of Zeus), and often portrayed them on bronze mirrors. Several places were named for the Dioscuri, including the Black Sea city of Dioscurias (modern Sokhumi), said to have been founded by them, the Libyan settlement of Dioscuron kome ("Village of the Dioscuri"), and the island the Greeks called Dioskouridou (Socotra). The Roman family name Ahenobarbus ("Brazen-bearded") was also explained through a legend in which the Dioscuri stroked an ancestor's black beard, turning it red, after announcing the victory at Lake Regillus.