The largest of the Ionian Islands, in the Ionian Sea west of the exit of the Gulf of Patras; it has given its name to territories that include more than the island:-
- A Byzantine theme;
- A Crusader county;
- A modern Greek nome or department.
It was a part of the Byzantine Empire: the theme of Cephalonia, formed in the early 9th century, was the military and civil administrative region for the whole Ionian Islands. Cephalonia was at times lost by Byzantium: for example, Robert Guiscard, Duke of Apulia, died there in 1085, having recently captured it. His death was followed by a Norman withdrawal.
Maio Orsini, an adventurer, was active in Cephalonia in the 1190s, and was recognised as Count by Venice in 1208. In 1324 the County became part of the Principality of Achaea, then subservient to the Angevin Kingdom of Naples. In 1357 it passed to the Tocco family, who also at times ruled in Acarnania and parts of Epirus on the mainland.
In 1483 Venice acquired Cephalonia and its smallish neighbour, Ithaca, but soon lost them to the Turks in 1485. They were recovered in 1500 and then remained Venetian until the fall of the Republic in 1797. With the rest of the Ionian Islands they passed from pillar to post after the fall of Venice, ending up as a British Protectorate in 1815 and as part of Greece in 1864.
The islands of Cephalonia and Ithaca now form the nome of Kefallinía, which belongs to the Ionian Islands region.