In the Roman mythology, the island was visited by Hercules.
The island was originally colonised by the Carians. A contingent from Kos participated in the War of Troy The Dorians invaded it in the 11th century BC, establishing a Dorian colony with a large contingent of settlers from Epidaurus, whose Asclepius cult made their new home famous for its sanatoria. The other chief sources of the island's wealth lay in its wines and, in later days, in its silk manufacture.
Its early history–as part of the religious-political amphictyony that included Lindos, Kamiros, Ialysos, Cnidus and Halicarnassus, the Dorian Hexapolis (Greek for six cities),–is obscure. At the end of the 6th century, Kos fell under Achaemenid domination but rebelled after the Greek victory at Cape Mykale in 479. During the Greco-Persian Wars, when it twice expelled the Persians, it was ruled by tyrants, but as a rule it seems to have been under oligarchic government. In the 5th century, it joined the Delian League, and, after the revolt of Rhodes, it served as the chief Athenian station in the south-eastern Aegean (411–407). In 366 BC, a democracy was instituted. After helping to weaken Athenian power, in the Social War (357-355 BC), it fell for a few years to the king Mausolus of Caria. In 366 BC, the capital was transferred from Astypalaia to the newly built town of Kos, laid out in a Hippodamian grid.
Proximity to the east gave the island first access to imported silk thread. Aristotle (384 BC-322 BC) mentions silk weaving conducted by the women of the island. Silk production of garments was conducted in large factories by women slaves.
In the Hellenistic age, Kos attained the zenith of its prosperity. Its alliance was valued by the kings of Egypt, who used it as a naval outpost to oversee the Aegean. As a seat of learning, it arose as a provincial branch of the museum of Alexandria, and became a favorite resort for the education of the princes of the