There is contradictory information regarding the Dioscuri's parentage. In Homer's Odyssey they are sons of Tyndareus alone, but in Hesiod's Catalogue of Women they were sons of Zeus. The conventional account, first attested in Pindar's Nemean Odes, combined these: only Pollux was fathered by Zeus, while Leda and Tyndareus conceived Castor. This explains their alternating immortality. The figure of Tyndareus may have entered the tradition to explain their archaic name — Tindaridai in Spartan inscriptions, Tyndaridai in literature — occasioning incompatible accounts of their parentage. Their sisters were Timandra, Phoebe, and Philonoe.

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

The Dioscuri were regarded as helpers of mankind and patrons of travelers and sailors in particular, who invoked them for favorable winds. Their roles as horsemen and boxers led to them being patrons of athletes and athletic contests. They characteristically intervened at moments of crisis, aiding those who honored or trusted them.

Ancient Greek authors tell numerous versions of Castor and Pollux's story. Homer portrays them initially as ordinary mortals, treating them as dead in the Iliad, where Helen states: "there are two commanders I do not see, / Castor the horse breaker and the boxer / Polydeuces, my brothers."

In the Odyssey they are described as both alive, even though "the grain-bearing earth holds them," and as having honor equal to the gods, living on alternate days because of Zeus's intervention. In both the Odyssey and Hesiod they are sons of Tyndareus and Leda; in Pindar, Pollux is the son of Zeus while Castor is the son of the mortal Tyndareus. The theme of ambiguous parentage appears similarly in the stories of Herakles and Theseus. The Dioscuri are invoked in Alcaeus's 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 of an ode to praising Castor and Pollux instead of Scopas's chariot-race victory. Shortly afterward Simonides was told that two young men wished to speak with him; after he left the banqueting room, the roof fell in and crushed Scopas and his guests. According to ancient sources, Castor's horse was named Cyllarus.

As Argonauts: Both Dioscuri were excellent horsemen and hunters who took part in hunting the Calydonian Boar and later joined Jason's ship, the Argo. During the voyage Pollux took part in a boxing contest and defeated King Amycus of the Bebryces, a savage people of Bithynia. After returning, the Dioscuri helped Jason and Peleus 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 brothers invaded his kingdom of Attica to rescue her. In revenge they abducted Theseus's mother Aethra and took her to Sparta, while setting 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.

Leucippides, Lynceus, and Death: Castor and Pollux aspired to marry the Leucippides ("daughters of the white horse"), Phoebe and Hilaeira, daughters of Leucippus. The 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 to Sparta, where each bore a son — Phoebe bore Mnesileos to Pollux and Hilaeira bore Anogon to Castor — beginning a family feud among the cousins.

The four cousins carried out a cattle-raid in Arcadia together but fell out over dividing the meat. The gigantic Idas proposed dividing the herd into two parts rather than four, awarding each half to whichever pair finished its portion first; he then quickly ate both his own and Lynceus's share, duping Castor and Pollux, who let the cousins take the whole herd but vowed revenge. Later, when Idas and Lynceus visited Sparta, Castor and Pollux seized the chance to steal their cousins' herd. While they were away, Paris of Troy — also a guest — carried off Helen, so the cousins' quarrel helped set the Trojan War in motion.

Castor climbed a tree to keep watch while Pollux freed the cattle. The sharp-eyed Lynceus spied Castor in the tree, and the furious Idas ambushed and fatally wounded him with a spear — but not before Castor warned Pollux. In the ensuing fight Pollux killed Lynceus, and as Idas was about to kill Pollux, Zeus hurled a thunderbolt that killed Idas and saved his son. Returning to the dying Castor, Pollux was offered by Zeus the choice of dwelling on Olympus forever or sharing half his immortality with his mortal brother; he chose the latter, so the twins alternate between Olympus and Hades. They became the two brightest stars of the constellation Gemini — Castor (Alpha Geminorum) and Pollux (Beta Geminorum) — and, like Heracles, were said to have been initiated into the Eleusinian Mysteries.

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 pieces of wood 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, and can be recognized in some vase-paintings by the skull-cap, the pilos, explained in antiquity as remnants of the egg from which they hatched. 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. Dares Phrygius described them as "blond haired, large eyed, fair complexioned, and well-built with trim bodies."

Dokana were symbolical representations of the Dioscuri consisting of two upright beams joined transversely. The twins were worshipped as gods of war, and their images accompanied Spartan kings on campaign; when in 504 BCE the two kings failed in an invasion of Attica through their mutual enmity, it was decreed that only one king should command the army, accompanied by only one of the twins' images. The word dokana seems to derive from dokos ("beam"), though the Suda and Etymologicum Magnum state it was the name of the Dioscuri's graves at Sparta.

The Dioskouroi were worshipped by Greeks and Romans alike, with temples in Athens (such as the Anakeion) and Rome and shrines in many other locations. The twins and their sisters grew up in Sparta in the royal household of Tyndareus, and they were particularly important to the Spartans, who associated them with the Spartan 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 in front of the army on campaign, and when the army marched to war one king remained at home accompanied by one of the twins.

Their herōon, or grave-shrine, was on a mountain top at Therapne across the Eurotas from Sparta, at a shrine known as the Menelaion, 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 and as deceased mortals in Hades whose spirits had to be propitiated with libations. The pear tree was sacred to them at Sparta, where images of the twins were hung in its branches, 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, and are sometimes shown arriving at a gallop over a food-laden table. The domestic setting of such offerings was a distinction accorded especially to the Dioskouroi. The image of the twins attending a goddess links them with male societies of initiates under the Anatolian Great Goddess and the great gods of Samothrace; they were venerated at Naukratis during the Archaic period and were said to be inventors of war dances. The Athenian festival in their honor was the Anakeia, the twins also bearing the name Anakes.

From the 5th century BCE onward the brothers were revered by the Romans, probably through cultural transmission via the Greek colonies of Magna Graecia. An archaic Latin inscription of the 6th or 5th century BCE from Lavinium reads "Castorei Podlouqueique qurois" ("To Castor and Pollux, the Dioskouroi"), suggesting direct transmission from the Greeks. The Etruscans venerated the twins as Kastur and Pultuce, collectively the tinas cliniiaras, "Sons of Tinia," and often portrayed them on mirrors.

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 Roman 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 July 15, the Feast Day of the Dioskouroi, 1,800 equestrians paraded through Rome in full military attire. In translations of Plautus's comedies, women generally swear by Castor and men by Pollux.

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 would not abandon. In some instances the twins were absorbed into a Christian framework, and the Church sought to replace them with equivalent Christian pairs — Saints Peter and Paul as patrons of travelers, and Saints Cosmas and Damian as healers. The heavenly twins also appear across Indo-European tradition as the Vedic Ashvins, the Lithuanian Ašvieniai, and possibly the Germanic Alcis.