Greek was not a uniform language; there were scores of dialects even within the tiny geographical area of mainland Greece. Greeks of different dialects could speak to and understand each other, with varying degrees of difficulty. Two dialects that were widely distant from each other might be just barely intelligible to each other. This would change over time. As the Greeks became more cosmopolitan, coming into wider and more frequent contact with each other and with other nations, the Greek language became more uniform. The development of literature and song also became a force for standardization, although never to the degree that we have seen for our own language under the influence of the printing press and television.

The principal divisions of Greek dialect are between Aeolic, Ionic, and Doric.

Attic-Ionic
This dialect group defines a series of closely related dialects spoken by Athenians and the Ionians occupying islands in the Aegean and the coast of modern day Turkey, but also includes several dialects spoken by peoples on the Aegean coast of the Greek mainland. This suggests that the Athenians are culturally related in some way to the Ionian Greeks. Due to the fact that Ionia and Athens are two of the greatest contributors to Greek cultural development (especially literature) and since they become, over time, the most economically successful, this group of dialects eventually becomes the most pervasive.
Most literature surviving from ancient Greece today is in the Attic or Ionic dialects. The Ionic dialect of Athens, Attic, is distinctive. Attic Greek is the language of the bulk of Golden Age Greek literature, and it is what students study when they begin to study Classical Greek.

Aeolic
This defines the dialects spoken by Greeks occupying the north and central islands of the Aegean, such as Lesbos. Sappho wrote many of her poems in her native Lesbian dialect, which is an Aeolic dialect. But Aeolic includes the dialects spoken by the Boeotians, northern neighbours of Attica, and the Thessalians above them.

Doric
This is the largest category of West Greek dialects, including those spoken by all the Greeks of the Peloponnesus except the Arcadians, Eleans, and some Achaians. The two most prominent Doric dialects are Argolic, spoken by the Argives of Argos, and Laconian, spoken by the Spartans. But Corinth, Megara, Rhodes, Cyrene, and Crete are all home to different Doric dialects as well.