Kefalonia or Cephalonia (Greek: Κεφαλονιά), formerly also known as Kefallinia or Kephallenia (Κεφαλληνία), is the largest of the Ionian Islands in western Greece and the 6th-largest island in Greece after Crete, Euboea, Lesbos, Rhodes and Chios. It is also a separate regional unit of the Ionian Islands region. It was a former Latin Catholic diocese Kefalonia–Zakynthos (Cefalonia–Zante) and short-lived titular see as just Kefalonia. The largest cities of Cephalonia are Argostoli and Lixouri.

An aition explaining the name of Cephallenia and reinforcing its cultural connections with Athens associates the island with the mythological figure of Cephalus, who helped Amphitryon of Mycenae in a war against the Taphians and Teleboans. He was rewarded with the island of Same, which thereafter came to be known as Cephallenia.

Kefalonia has also been suggested as the Homeric Ithaca, the home of Odysseus, rather than the smaller island bearing this name today. Robert Bittlestone, in his book Odysseus Unbound, has suggested that Paliki, now a peninsula of Cephalonia, was a separate island during the late Bronze Age, and it may be this which Homer was referring to when he described Ithaca. A project that started in the summer of 2007 and lasted three years has examined this possibility.

Kefalonia is also referenced in relation to the goddess Britomartis, as the location where she is said to have 'received divine honours from the inhabitants under the name of Laphria'.

From at least the 6th century BC, the island was dominated by four city-states (poleis): Pale (modern Lixouri), Cranii (mod. Argostoli), Same (mod. Sami) and Pronnoi. All four minted their own coins, as well as building monumental temples and fortifications, both in the cities themselves and in the surrounding countryside. Ancient writers generally paid little attention to the island throughout antiquity, but there are some notable references, and it seems that the Kefalonian cities were involved in developments and events across the wider Greek world.

A certain Melampous, from Kefalonia, won the Lyre and Song contest at the Pythian Games at Delphi in 582 BC. 200 hoplites from Pale fought alongside other Greeks against the Persians in the decisive battle at Plataea, and all four Kefalonian cities allied with Athens during the Peloponnesian War. The island was of strategic value to the Athenians, as it lies close to the entry to the Bay of Corinth. The Corinthians attempted, unsuccessfully, to attack Krane in 431 BC, and, 10 years later, Athens settled a group of Spartan deserters on the island. Finally, a group of Kefalonian soldiers were recruited by the Athenian general Demosthenes as part of the ill-fated Sicilian Expedition in 415-413 BC.

According to Strabo, on Mount Ainos there was a sanctuary dedicated to Zeus Ainesios. This temple was also mentioned by Hesiod, and German, French and English explorers in the 18th and 19th century found its remnants.

The Kefalonian cities retained close ties with Athens after the end of the Peloponnesian War, despite the Spartan victory. Athenian influence, including heavy direct taxation in the 4th century BC, may have been a stimulus for a new planned and fortified town at Same, which increasingly seems to have dominated the smaller polis of Pronnoi.