Kos was inhabited from the very early Bronze Age (2900-2100 BC) as the prehistoric tombs and the findings in Asklupi and in the White Stone cave prove. Pelasgi, Kares and Leleges were the first inhabitants. Phoenicians and Achaeans passed through Kos as well.

The Hellenistic period is the brightest period in Kos' history. In the creation of the new city (366 BC) a lot of marble monuments were built such as the sanctuary of Hercules, Pandimou and Pondias Aphrodite, the Market, the Gymnasium, the Stadium, the Theatre, the Altar of Dionysus and the Acropolis.

Kos prospered during the late Hellenistic period. Elegant buildings such as the conservatory, the Casa Romana, the Vespasiani ("Nimphea"), the Thermes with it's rich mosaics, all demonstrate the luxury in which the citizens of the island lived. The harbour of Kos never stopped being the centre of transit trade.