Kos or Cos (; Greek: Κως ) is a Greek island, part of the Dodecanese island chain in the southeastern Aegean Sea. Kos is the third largest island of the Dodecanese by area, after Rhodes and Karpathos; it has a population of 33,388 (2011 census), making it the second most populous of the Dodecanese, after Rhodes. The island measures 40 by 8 kilometres (25 by 5 miles). Administratively, Kos constitutes a municipality within the Kos regional unit, which is part of the South Aegeanregion. The principal town of the island and seat of the municipality is Kos Town.