Castor and Pollux
In Greek mythology the Dioskouroi (),  Kastor and Polydeuces (  ), in Roman mythology  the Gemini (Latin, "twins") or Castores, Castor and Pollux are the twin sons of Leda and the brothers of Helen of Troy and Clytemnestra.  According to Liddell and Scott's Lexicon, kastor is Greek for "he who excels", and poludeukeis means "very sweet".

# Origins
They are called the Dioscuri (dios kouroi), meaning the "youths of Zeus". Their Vedic parallels in the effulgent brother horsemen Asvin sets them firmly in the Indo-European tradition (Burkert 1985:212). Their archaic and inexplicable name in Spartan inscriptions Tindaridai or in literature Tyndaridai occasioned an explanatory myth of a Tyndareus (Burkert 1985:212), occasioning incompatible accounts of their parentage, as that for their sisters Helen and Clytemnestra. The better known story is that Zeus disguised himself as a swan and seduced Leda.  Thus Leda's children are frequently said to have hatched from two eggs that she then produced. The Dioscuri can be recognized in 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.  Tyndareus, Leda's mortal husband, is then father or foster-father to the children.  Whether the children are thus mortal and which half-immortal is not consistent among accounts, nor is whether the twins hatched together from one egg. In some accounts, only Polydeuces was fathered by Zeus, while Leda and her husband Tyndareus conceived Castor.  This explains why they were granted an alternate immortality. It is a common belief that  one  would live among the gods, while the other  was among the dead.  They do make an appearance together in the play, Helen and Electra.
Castor and Polydeuces are sometimes both mortal, sometimes both divine.  One consistent point is that if only one of them is immortal, it is Polydeuces.  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, thus suggesting that at least in some early traditions, both were mortal. Their death and shared immortality offered by Zeus was material of the lost Cypria in the Epic cycle.
As a further complication, the Zeus-as-swan myth is sometimes associated with the goddess Nemesis.  In this tradition, it was Nemesis who was seduced and who laid the egg, but the egg was then found by or given to Leda.  However, this story is usually associated with Helen, ordained by Zeus to cause the Trojan War, and not with Castor and Polydeuces.

# Connections with Sparta
The Dioscuri and their sisters grew up in Sparta, in the household of Tyndareus (see above). Their connection there was very ancient: a uniquely Spartan aniconic representation of the Tyndaridai was as two upright posts joined by a cross-bar. Sparta's unique dual kingship reflects the divine influence of the Dioscuri. When the Spartan army marches to war, one king remains behind at home, accompanied by one of the Twins. "In this way the real political order is secured in the realm of the Gods" (Burkert 1985:212).
Their heroon or grave-shrine was at Therapne across the Eurotas from Sparta.

# Dioscuri as adventurers
They accompanied Jason on the Argo; during the voyage, Polydeuces killed King Amycus in a boxing match.
When Astydameia, queen of Iolcus, offended Peleus, the twins assisted him in ravaging her country.

# Dioscuri as saviours
When Theseus and Pirithous kidnapped their sister Helen and carried her off to Aphidnae, the twins rescued her and counter-abducted Theseus' mother, Aethra.  The mounted horsemen who rode out to save their abducted sister Helen from Theseus could be expected to show up to succour their votaries, as when the Locrians of Magna Graecia attributed their success at a legendary battle on the banks of the Sagras to the intervention of the Twins.

# Dioscuri in the service of the Goddess
The image of the twins attending a goddess are widespread and link the Dioscuri with the male societies of initiates under the aegis of the  Anatolian Great Goddess and the great gods of Samothrace. The Dioscuri are the inventors of war dances, which characterize the Kuretes.

# Mortality and immortality
Castor and Polydeuces abducted the Leucippides ("white horses") Phoebe and Hilaeira, the daughters of Leucippus (mythology).  When they encountered their analogous twin brothers of Thebes, Idas and  "lynx-eyed" Lynceus, bound for revenge, Castor, the mortal brother, fell, and Polydeuces, the immortal twin, survived, yet they were not separated. Polydeuces persuaded Zeus to share his gift with Castor. Accordingly, the two spend alternate days as gods on Olympus, worthy of burnt sacrifice, and as deceased mortals in Hades, whose spirits must be propitiated by libations.
The lost Cypria explained the terms of their joint immortality as a gift of Zeus. In Odyssey, Homer renders the paradox:
As emblems of immortality and death that were no longer polar opposites, it is not surprising to hear that the Dioscuri, like Heracles were said to have been initiated at Eleusis.

# Roman Castor and Pollux
As early as 484 BCE, a temple to the Castores was erected in the Roman Forum in gratitude for their supposed intervention in the Battle of Lake Regillus. Their festival was celebrated on July 15.
Following tradition as old as Homer, Pollux was accounted a powerful boxer, and Castor a great horseman. In Roman mythology, Castor was venerated much more often than Pollux, to the extent that the pair became known as the Castores.
For other examples of the mytheme of the Unequal Twins, compare Amphion and Zethus of Thebes and Romulus and Remus of Rome. Compare also the Alcis of Germanic Mythology with the Asvins of Vedic mythology, suggesting an Indo-European origin for the myth of the divine twins.

# In astronomy
The constellation Gemini is said to represent these twins.  Its brightest stars, Castor and Pollux ( and  Geminorum), are named for them.

# In Religion
Acts 28:11- And after three months we departed in a ship of Alexandria, which had wintered in the isle, whose sign (figurehead) was Castor and Pollux (the Twin Brothers of Zeus).

# In art
Various classical sculptural groups of 2 nude males have been identified as Castor and Pollux - sometimes securely (eg the Castor and Pollux at the Prado Museum), sometimes less so (eg The Horse Tamers) .
Castor et Pollux was the title of a 1737 opera by Jean-Philippe Rameau (libretto by Bernard), modified in 1754. The latter version became quite popular.

# In popular culture
- In the movie Face/Off, made in 1997, Nicolas Cage and Alessandro Nivola play brothers Castor and Pollux Troy, a pair of terrorists-for-hire who are devoted to each other even though they are deeply psychotic and paranoid.
- In 1842 Lord Macaulay wrote a series of poems about Ancient Rome (the Lays of Ancient Rome).  The second poem is about the Battle of Lake Regillus and describes the intervention of Castor and Pollux.  They are referred to as the Great Twin Brethren in the poem. 

# Notes
-  Scholiast on Lycophron, noted by Karl Kerenyi, 1959. The Heroes of the Greeks p.107 note 584.
-  The familiar theme in Greek mythology of the mixed seed of a mortal and an immortal father is played out in various ways: compare Theseus.
-  Burkert 1985; Kerenyi 1959:107)
-  Kerenyi 1959 draws attention espercially to the rock carvings in the town of Akrai, Sicily (1959:111).
-  Burkert 1985:212, who notes F. Chapouthier, Les Dioscures au service d'une deesse, 1935.
-  The reader will immediately recognize in Phoebe ("the pure") an epithet of the moon, Selene; her twin's name Hilaeira ("the serene") is also a lunar attribute, their names "appropriate selectively to the new and the full moon" (Kerenyi 1959:109).
-  In the oration of the Athenian peace emissary sent to Sparta in 371, according to Xenophon (Hellenica VI), it was asserted that "these three heroes were the first strangers upon whom this gift was bestowed." (Karl Kerenyi, 1967. Eleusis: Archetypal Image of Mother and Daughter (Princeton: Bollingen), p. 122.
-  "Castor, the breaker of horses, and Polydeuces the hardy boxer" (Odyssey XI.300