In the myth the twins shared the same mother but had different fathers which meant that Pollux was immortal and Castor was mortal. When Castor died, Pollux asked Zeus to let him share his own immortality with his twin to keep them together and they were transformed into the Gemini constellation. The pair were regarded as the patrons of sailors, to whom they appeared as Saint Elmo's Fire.
When their sister Helen was abducted by the legendary Greek king Theseus, they invaded his kingdom of Attica to rescue her, abducting Theseus' mother Aethra in revenge and carrying her off to Sparta while setting a rival, Menestheus, on the throne of Athens. Aethra was forced to become Helen's slave but was eventually returned to her home by her grandsons Demophon and Acamas following the fall of Troy.
Castor and Pollux aspired to marry the Leucippides, Phoebe and Hilaeira, the daughters of their uncle Leucippus. Although both women were already betrothed to Lynceus and Idas, the sons of Aphareus, the twins carried them both off to Sparta where Phoebe bore Mnesileos to Pollux and Hilaeira bore Anogon to Castor. This began a feud between the four cousins. They carried out a cattle-raid in Arcadia together but fell out over the division of the meat, prompting the Dioskouroi to seize all the cattle and drive them back to Sparta, pursued by Lynceus and Idas. Castor was ambushed by Idas and fatally wounded by a blow from his cousin's spear, but Idas was himself killed by a thunderbolt hurled in revenge by Zeus while Lynceus was killed by Pollux.
Returning to the dying Castor, Pollux was given the choice by Zeus of spending all his time on Mount Olympus or giving half his immortality to his mortal brother. He opted for the latter, enabling the twins to alternate between Olympus and Hades. The brothers became part of the stellar constellation Gemini ("the twins"), becoming the two brightest stars in the group: Castor (Alpha Geminorum) and Pollux (Beta Geminorum).
Castor and Pollux are constantly associated with horses in art and literature. They bear striking similarities in this respect to divine twins in other mythologies, especially the Vedic Ashvins, who like them have a close association with horses. Their role as horsemen made them particularly attractive to the Roman equites and cavalry. Each year on July 15, the feast day of the Dioskouroi, the 1,800 equestrians would parade through the streets of Rome in an elaborate spectacle in which each rider wore full military attire and whatever decorations he had earned.
The twins were widely depicted as helmeted horsemen carring spears. The Pseudo-Oppian manuscript depicts the brothers hunting, both on horseback and on foot. On votive reliefs they are depicted with a variety of symbols representing the concept of twinhood, such as the dokana (δόκανα - two upright piece of wood connected by two cross-beams), a pair of amphorae, a pair of shields, or a pair of snakes. They are also often shown wearing felt caps, above which stars may be depicted. They are depicted on metopes from Delphi showing them on the voyage of the Argo (Ἀργώ) and rustling cattle with Idas. Greek vases regularly show them in the rape of the Leucippides, 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 (πῖλος), which was already explained in Antiquity as the remnants of the egg from which they hatched.
The Dioskouroi were worshipped by the Greeks and Romans alike; there were temples to the twins in Athens and Rome as well as shrines in many other locations in the ancient world. They were particularly important to the Spartans, who associated them with the Spartan tradition of dual kingship. They were seen as the protectors of the Spartan army, and the "beam figure" or dokana associated with Castor and Pollux was carried in front of the army on campaign. The Spartans erected a shrine known as the Menelaion on a mountain top at Therapne where Helen, Melelaus, Castor and Pollux were all said to be buried. They were commemorated both as gods on Olympus worthy of burnt sacrifice, and as deceased mortals in Hades, whose spirits had to be propitiated by libations. Lesser shrines to Castor, Pollux and Helen were also established at a number of other locations around Sparta. The pear tree was regarded by the Spartans as sacred to Castor and Pollox, and images of the twins were hung in its branches.
From the fifth century BC onwards, the brothers were revered by the Romans, probably as the result of cultural transmission via the Greek colonies of Magna Graecia in southern Italy. An archaic Latin inscription of the sixth or five century BC found at Lavinium, which reads Castorei Podlouqueique qurois ("To Castor and Pollux, the Dioskouroi"), suggests a direct transmission from the Greeks; the word "qurois" is virtually a transliteration of the Greek word κούροις, while "Podlouquei" is effectively a transliteration of the Greek Πολυδεύκης. The Romans believed that the twins aided them on the battlefield. The construction of the Temple of Castor and Pollux, located in the Roman Forum at the heart of their city, was undertaken to fulfil a vow sworn by Aulus Postumius Albus Regillensis in gratitude at the Roman victory in the Battle of Lake Regillus in 495 BC. According to legend, the twins fought at the head of the Roman army and subsequently brought news of the victory back to Rome. In a very similar vein, the Locrians of Magna Graecia attributed their success at a legendary battle on the banks of the Sagras to the intervention of the Twins. The Roman legend may in fact have had its origins in the Locrian account and possibly supplies further evidence of cultural transmission between Rome and Magna Graecia.
The Celts also worshipped Castor and Pollux; the 1st century BC historian Diodorus Siculus records that the twins were the gods most worshipped in the west of Gaul. An altar found at Paris depicts them among Celtic figures such as the god Cernunnos. Elsewhere in Italy the twins were also venerated by the Etruscans, who knew them as Kastor and Poltuce, collectively the tinas cliniiaras ("sons of Tinia [Zeus]"). They were often portrayed on Etruscan mirrors. As was the fashion in Greece, they could also be portrayed symbolically; one example can be seen in the Tomba del Letto Funebre at Tarquinia where a lectisternium for them is painted. They are symbolised in the painting by the presence of two pointed caps crowned with laurel, referring to the Phrygian caps which they were often depicted as wearing.
The Dioskouroi 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. Cicero tells the story of how Simonides of Ceos was rebuked by Scopas, his patron, for devoting too much space to praising Castor and Pollux in an ode celebrating Scopas' victory in a chariot-race. Shortly afterwards, Simonides was told that two young men wished to speak to him; after he had left the banqueting room, the roof fell in and crushed Scopas and his guests.
The rite of theoxenia (θεοξενία), "god-entertaining", was particularly associated with Castor and Pollux. The two deities were summoned to a table laid with food, whether at individuals' own homes or in the public hearths or equivalent places controlled by states. They are sometimes shown arriving at a gallop over a food-laden table. Although such "table offerings" were a fairly common feature of Greek cult rituals, they were normally made in the shrines of the gods or heroes concerned. The domestic setting of the theoxenia was a characteristic distinction accorded to the Dioskouroi.
Even after the rise of Christianity, the Dioskouroi continued to be venerated. The fifth-century pope Gelasius I attested to the presence of a "cult of Castores" that the people did not want to abandon. In some instances, the twins appear to have simply been absorbed into a Christian framework; thus fourth-century AD pottery and carvings from North Africa depict the Dioskouroi alongside the Twelve Apostles, the Raising of Lazarus or with Saint Peter. The church took an ambivalent attitude, rejecting the immortality of the Dioskouroi but seeking to replace them with equivalent Christian pairs. Saints Peter and Paul were thus adopted in place of the Dioskouroi as patrons of travellers, and Saints Cosmas and Damian took over their function as healers. Some have also associated Saints Speusippus, Eleusippus, and Melapsippus with the Dioskouroi.