The Aetolian (or Aitolian) League (Κοινὸν τῶν Αἰτωλῶν) was a confederation of tribal communities and cities in ancient Greece centered in Aetolia in Central Greece. It was probably established during the early Hellenistic era, in opposition to Macedon and the Achaean League. Two annual meetings were held at Thermika and Panaetolika. The league occupied Delphi from 290 BC and steadily gained territory until, by the end of the 3rd century BC, it controlled the whole of central Greece with the exception of Attica and Boeotia. At its peak, the league's territory included Locris, Malis, Dolopes, parts of Thessaly, Phocis, and Acarnania. In the latter part of its power, certain Greek city-states joined the Aetolian League such as the Arcadian cities of Mantineia, Tegea, Phigalia and Kydonia on Crete.
During the classical period the Aetolians were not highly regarded by other Greeks, who considered them to be semi-barbaric and reckless. Their League had a complex political and administrative structure, and their armies were easily a match for the other Greek powers. However, during the Hellenistic period, they emerged as a dominant state in central Greece and expanded by the voluntary annexation of several Greek city-states to the League. Still, the Aetolian League had to fight against Macedonia and were driven to an alliance with Rome, which resulted in the final conquest of Greece by the Romans.
The Aetolians were a recognised ethnic group with a religious centre at Thermos from at least the seventh century BC.
During the Peloponnesian War, the Aetolians were initially neutral, but when the Athenians tried to invade Aetolia in 426 BC, the Aetolians forced them to retreat. In the course of the fourth century, the league offered passive support to more powerful states and was rewarded for it, receiving Aeolis from the Thebans in 367 BC and Naupactus from Philip II of Macedon in 338 BC. Sometime in this century, the Koinon tōn Aitōlōn (League of the Aetolians) was founded, but it is uncertain when.