The Achelous, also Acheloos, is a river in Epirus, western Greece. It is 220 km (137 mi) long. It formed the boundary between Acarnania and Aetolia of antiquity. It empties into the Ionian Sea. In ancient times its spirit was venerated as the river god Achelous.

Aetolia-Acarnania is one of the regional units of Greece. It is part of the geographic region of Central Greece and the administrative region of West Greece. A combination of the historical regions of Aetolia and Acarnania, it is the country's largest regional unit. Its capital is Missolonghi for historical reasons, with its biggest city and economic centre at Agrinio. The area is now connected with the Peloponnese peninsula via the Rio-Antirio Bridge. The surrounding regional units take in Arta in Epirus, a narrow length bordering Karditsa of Thessaly, Evrytania to the northeast, and Phocis to the east.