Polycleitus was a sculptor of Greek antiquity, and one of the most famous at that. He was born in Argos at some time during the second half of the fifth century BCE and his statues graced several locations throughout Greece. His work included nude statues of several athletes in their perfect forms for Olympia, religious carvings of the gods such as a gold and ivory cast of Hera, and the individual pieces concerning poses from youth for which he is most famous in modern days.

The most of the actual original works of Polycleitus were unfortunately destroyed through various tragic events, however we still know much about his sculpture and style. The Romans, ever modeling their artwork on the more culturally sophisticated Greeks, copied several of his sculptures for their own display. One such reproduction is the statue Doryphorus (youth holding a spear) which reflects Polycleitus intuitive sense for harmony of form and staggering detail. Other finds include just the bases of aforementioned athlete statues. He also had an admirer in Pliny the Elder, who wrote extremely detailed accounts of many of the statues of which we no longer have relics or reproductions, completing the picture.

Howatson, M. C. The Oxford Companion to Classical Literature. Oxford: Oxford University Press, 1989.
Yahoo Reference: Polyclitus, http://education.yahoo.com/reference/dictionary/entries/23/p0422300.html
Images from World History: Greece, http://www.hp.uab.edu/image_archive/ulc/ulce.html