The Castor and Pollux group (also known as the San Ildefonso Group, after San Ildefonso in Segovia, Spain, the location of the palace of La Granja at which it was kept until 1839) is an ancient Roman sculptural group of the 1st century AD, considered one of the most outstanding examples of neo-Attic eclecicism. This is a scan of a plaster cast of the original marble housed at The Museo del Prado, Madrid

Despite drawing on strong influences from 5th and 4th century BC Greek sculptures in the Praxitelean tradition, such as the Apollo Sauroctonos or the 'Westmacott Ephebe', the marble sculpture does not copy any single known Greek work. The group shows two idealised nude youths, both wearing laurel wreaths. The young mean lean against each other, and to their left on an altar is a small femmale figure - generally interpreted as a statue of a female divinity. She holds a sphere, variously interpreted as an egg or pomegranate.

The identification of the two figures has gone under much debate but is now globally accepted as portraying Castor and Pollux. The lefthand figure (when facing the sculpture) was originally headless upon its discovery but was restored in the 17th century, when interpretive restorations was commonplace, by Ippolito Buzzi in the collection of Cardinal Ludovico Ludovisi. A Hadrianic-era (ca. 130) bust of Antinous of the Apollo-Antinous type from another statue was used in the restoration.

During the 17th and 18th centuries, various theories to the identification of the two figures were raised. During the 19th Century the work became known as "Antinous and Hadrian's genius", to get over the problem of them both being youths, whereas ahistorically it was an important feature of Antinous' relationship with Hadrian that Antinous was a youthful eromenos and Hadian an elder erastes. Anternatively "Antinous and a sacrificial daemon" was suggested, in reference to the myth that Antinous had killed himself as a sacrifice to lengthen Hadrian's life, or simply as Andinous and Hadrian pledging their fidelity to one another.

All these identifications are now thought to be erroneous and simply due to the figure's restoration as Antinous: the group is now accepted as Castor and Pollux, offering a sacrifice to Persephone. Such an identification is based on the right-hand figure, who holds two torches, one downturned (on a flower-wreathed altar) and one upturned (behind his back), and on identifying the woman's sphere as an egg (like that from which the Dioscuri were born). The interpretation was supported by Goethe, who owned a cast of the group.

In Greek and Roman mythology, Castor and Pollux (or Kastor and Polydeuses), were twin brothers, together known as the Dioscuri or Dioskouroi. Their mother was Leda, but they had different fathers; Castor was the mortal son of Tyndareus, the king of Sparta, while Pollux was the divine sone of Zeus, who seduced Leda in the guise of a swan.

In Latin the twins are also known as the Gemini or Castores. When Castor was killed, Pollux asked Zeus to let him share his own immortality with his twin to keep them together, and they were transformed into the constellation Gemini. The pair were regarded as the patrons of sailors, to whom they appeared as St. Elmo's fire, and were also associatedwith horsemanship.