From excavations finds in the cave of Aspri Petra near the town
of Kefalos we find that Kos was inhabited from prehistoric
times. New researches placed these finds of Aspri Petra in the
Early Bronze Age, known as the Proto Helladic period 2900-2100
Following the Trojan War, in which Kos together with other nearby islands participated the Trojan War with 30 ships. During the 7th and 6th century BC, Kos formed the Dorian League together with the cities of Halicarnassus and Knidus in Asia Minor and Ialyssos , Lindos and Kameiros of Rhodes ,that league was known as the Doric Hexapolis.
The Apollo Triopius at Knidus was center of this league of towns. The League had economic ,political and religious character.
At 5th century BC the island of Kos was occupied by the Persians and
therefore Kos was forced to participate with them at the Battle of
Salamis (480 BC). Following the victory of the Greeks against the
Persians ,Kos was liberated and joined the Athenian League.
Hippocrates (460-377 BC) the father of medicine and one of the famous persons in the Greek history, was born on Kos, Hippocrates collected information and studies during his travels ,after his return to Kos established his Medical School .Hippocrates scientific works known as the Hippocratic Collection, still studied today in the Universities. Kos was occupied briefly by the Persians after the Peloponnesian wars and liberated by Alexander the Great in 332 BC.
The island had many rulers from the Hellenistic period ,including the Romans and later the Venetians, Genovese, Turks ,and finally the Italians until 1948 when Kos was unified with Greece.