The Haliacmon is the longest river flowing entirely in Greece, with a total length of 297 km (185 mi). In Greece there are three rivers longer than Haliakmon, Maritsa, Struma (Strymónas), both coming from Bulgaria, and Vardar coming from North Macedonia, but the length of each one of them in Greek territory is less than that of Haliakmon, which flows entirely in Greece. Haliacmon is the traditional English name for the river, but many sources cite the formerly official Katharevousa version of the name, Aliákmon. Today, the only official variant is the demotic Aliákmonas. It flows through the Greek regions of West Macedonia and Central Macedonia. Its drainage basin is 8,813 km2 (3,403 sq mi).

Western Macedonia is one of the thirteen administrative regions of Greece, consisting of the western part of Macedonia. Located in north-western Greece, it is divided into the regional units of Florina, Grevena, Kastoria, and Kozani. With a population of approximately 255,000 people, as of 2021, the region had one of the highest unemployment rates in the European Union.