After the decline of Minoan Crete, the Achaians arrived at Kos. According to Homer, Kos, along with Nisyros, Karpathos, and Kalymnos played an important part in the Trojan war. After the fall of Troy, Podarios, son of the God Asklepios and doctor from Thesaly, settled on the island after being shiprecked. Podarios created the family of the Asklipidai, of which Hippocrates, the father of medicine, is the 18th decendant.

In the 11th century B.C. the Dorians invaded Kos and expelled the Achaians. In the 7th century B.C. Kos entered a federation with six other cities in Asia Minor, Rhodes, Kalymnos and Nisyros. At the end of the 6th century, Kos, as did the other cities of Asia Minor was subdued by Darius, King of the Persians. In 479 B.C. the island was liberated by the united Greek army after the battle of Salamina where the Persian King Xerxes was defeated.

During the Peloponnesian war (431-404 B.C.) Kos allied with Athens. For this the island paid a high tribute when the Spartan Commander Astochos invaded the island in 411 B.C. In 394 B.C. and after a treaty with Sparta the Koans once again allied with Athens. Under the influence of Athens democracy was introduced. The level of culture, education, and economy during this period is very high. This continued until King Mansolos from Halikarnasos conquered the island in 358 B.C.

In 334 B.C. the island allied with Macedonia and Alexander the Great.

After 82 B.C. Kos became a part of the Eastern colony of the Roman Empire and was granted special privileges. These priveleges where lost during the reign of August, causing a period of stagnation. The decline was followed by a terrible earthquake in 27 B.C.

During the Byzantine period Kos flourished as part of the Byzantine empire. Due to its geographic position the island was continuously attacked and subjected to the blind fanatism of the Arabs and Turks. In 1204 A.D. the island was occupied by the Venetians. In 1457 A.D. the island was looted by a powerful Turkish army. This lasted a short time due to the help of allies of the island. After repeated attacks, the Turks conquered the island which was finally subdued by Sultan Suleyman.

During the Turkish oppression the Koans never gave up their heroic resistance.

On May 5th, 1912 Italian troups invaded the island and expelled the Turks. In 1934 an earthquake destroyed 80% of the island. The occupation was continued by the Germans in 1943, with a short interval of 20 days when the Englishman Batalos ruled. During 18 months the Koans suffered unhumanly until 1945 when the island came under British control.

On March 7th, 1948 Kos was returned to Greece.