Kea (Greek: Κέα), also known as Gia or Tzia (Greek: Τζια), Zea, and, in antiquity, Keos (Greek: Κέως, Latin: Ceos), is a Greek island in the Cyclades archipelago in the Aegean Sea. Kea is part of the Kea-Kythnos regional unit. Its capital, Ioulis, is inland at a high altitude (like most ancient Cycladic settlements, for fear of pirates) and is considered quite picturesque.
Kea is the location of a bronze-aged settlement at the site now called Ayia Irini, which reached its height in the Late Minoan and Early Mycenaean eras (1600-1400 BCE).
During the classical period, Kea (Ceos) was the home of Simonides and of his nephew Bacchylides, both ancient Greek lyric poets, of the Sophist Prodicus, and of the physician Erasistratus. The inhabitants were known for offering sacrifices to the Dog Star, Sirius and to Zeus to bring cooling breezes while awaiting for the reappearance of Sirius in summer; if the star rose clear, it would portend good fortune; if it was misty or faint, then it foretold (or emanated) pestilence. Coins retrieved from the island from the 3rd century BC feature dogs or stars with emanating rays, highlighting Sirius’ importance.
During the Byzantine period, many churches were built and the prosperity of the island rose. It was Byzantine until, in 1204, it was captured by the Venetians in the wake of the Fourth Crusade. The Archbishop of Athens, Michael Choniates, came here in exile after his city fell to the Crusaders in 1205. It was recaptured by the Byzantines under Licario in 1278. In ca. 1302 during the Byzantine–Venetian War, it fell to the Venetians again, who built a castle on the ancient acropolis of Ioulis. In 1330 Cos became the seat of a Latin Church bishop but, since it is no longer a residential bishopric, it is today listed by the Catholic Church as a titular see. The earliest indication of it as a Greek bishopric is in a list by the Sicilian monk Nil Doxapatris of the second half of the 12th century and this may have been a later interpolation, since the list of the Greek bishops of Kea begins only at the end of the 16th century.