Ancient Greek (Ἑλληνική, Hellēnikḗ [hellɛːnikɛ́ː]) includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (c. 1400–1200 BC), Dark Ages (c. 1200–800 BC), the Archaic or Homeric period (c. 800–500 BC), and the Classical period (c. 500–300 BC).

Ancient Greek was the language of Homer and of fifth-century Athenian historians, playwrights, and philosophers. It has contributed many words to English vocabulary and has been a standard subject of study in educational institutions of the Western world since the Renaissance. This article primarily contains information about the Epic and Classical periods of the language, which are the best-attested periods and considered most typical of Ancient Greek.

From the Hellenistic period (c. 300 BC), Ancient Greek was followed by Koine Greek, which is regarded as a separate historical stage, though its earliest form closely resembles Attic Greek, and its latest form approaches Medieval Greek, and Koine may be classified as Ancient Greek in a wider sense – being an ancient rather than medieval form of Greek, though over the centuries increasingly resembling Medieval and Modern Greek.

Ancient Greek comprised several regional dialects, such as Attic, Ionic, Doric, Aeolic, and Arcadocypriot; among them, Attic Greek became the basis of Koine Greek. Just like Koine is often included in Ancient Greek, conversely, Mycenaean Greek is usually treated separately and not always included in Ancient Greek – reflecting the fact that Greek in the first millennium BC is considered prototypical of Ancient Greek.

Ancient Greek was a pluricentric language, divided into many dialects. The main dialect groups are Attic and Ionic, Aeolic, Arcadocypriot, and Doric, many of them with several subdivisions. Some dialects are found in standardized literary forms in literature, while others are attested only in inscriptions.

There are also several historical forms. Homeric Greek is a literary form of Archaic Greek (derived primarily from Ionic and Aeolic) used in the epic poems, the Iliad and the Odyssey, and in later poems by other authors. Homeric Greek had significant differences in grammar and pronunciation from Classical Attic and other Classical-era dialects.

The origins, early form and development of the Hellenic language family are not well understood because of a lack of contemporaneous evidence. Several theories exist about what Hellenic dialect groups may have existed between the divergence of early Greek-like speech from the common Proto-Indo-European language and the Classical period. They have the same general outline but differ in some of the detail. The only attested dialect from this period is Mycenaean Greek, but its relationship to the historical dialects and the historical circumstances of the times imply that the overall groups already existed in some form.

Scholars assume that major Ancient Greek period dialect groups developed no later than 1120 BC, at the time of the Dorian invasions—and that their first appearances as precise alphabetic writing began in the 8th century BC. The invasion would not be "Dorian" unless the invaders had some cultural relationship to the historical Dorians. The invasion is known to have displaced population to the later Attic-Ionic regions, who regarded themselves as descendants of the population displaced by or contending with the Dorians.