Kos wurde bereits in der sehr fruehen frühen Bronzezeit (2900-2100 v. Chr.) bewohnt, wie die prähistorischen Gräberm die Entdeckungen in Asklupi und in der weißen Steinhöhle beweisen. Die Pelasger, Kares und die Leleger waren die ersten Einwohner, aber auch die Phönizier und Acheans durchkreuzten

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.