Cephalonia or Kefalonia, formerly also known as Kefallinia or Kephallenia, is the largest of the Ionian Islands in western Greece and the 6th largest island in Greece after Crete, Evoia, Lesbos, Rhodes, and Chios. It is also a separate regional unit of the Ionian Islands region, and the only municipality of the regional unit. It was also a former Latin Catholic diocese Kefalonia-Zakynthos (Cefalonia-Zante) and short-lived titular see as just Kefalonia.

The capital of Cephalonia is Argostoli.

Tourism to Cephalonia started in the early 19th century. The royal family of Greece sent their children in the summer months to Lixouri, in the early 20th century, but the island was not discovered by most tourists until the 1980s. Cephalonia is a popular vacation destination for many Italians, due to its proximity to Italy.

Two cultural attractions, the fishing villages of Fiscardo and Assos, and other natural attractions, including Melissani underground lake, Drogarati cave and Myrtos beach, have helped popularize Cephalonia. The film, Captain Corelli's Mandolin (2001), shot on the island itself, made Cephalonia more widely known.

Listed in The Daily Telegraph as one of the 20 Mediterranean islands you must visit in your lifetime.