The Ardennes, also known as the Ardennes Forest or Forest of Ardennes, is a region of extensive forests, rough terrain, rolling hills and ridges primarily in Belgium and Luxembourg, extending into Germany and France.

Geologically, the range is a western extension of the Eifel; both were raised during the Givetian age of the Devonian (382.7 to 387.7 million years ago), as were several other named ranges of the same greater range.

The Ardennes proper stretches well into Germany and France (lending its name to the Ardennes department and the former Champagne-Ardenne region) and geologically into the Eifel (the eastern extension of the Ardennes Forest into Bitburg-Prüm, Germany); most of it is in the southeast of Wallonia, the southern and more rural part of Belgium (away from the coastal plain but encompassing more than half of the country's total area). The eastern part of the Ardennes forms the northernmost third of the Grand Duchy of Luxembourg, also called "Éislek" (German: Oesling). On the southeast the Eifel region continues into the German state of the Rhineland-Palatinate.

The trees and rivers of the Ardennes provided the charcoal industry assets that enabled the great industrial period of Wallonia in the 18th and 19th centuries, when it was arguably the second great industrial region of the world. The greater region maintained an industrial eminence into the 20th century, after coal replaced charcoal in metallurgy. The strategic position of the Ardennes has made it a battleground for European powers for centuries; it was the site of major battles during both World Wars.

Much of the Ardennes is covered in dense forests, with the hills averaging around 350–400 m (1,150–1,310 ft) in height but rising to over 694 m (2,277 ft) in the boggy moors of the High Fens region of south-eastern Belgium. The region is typified by steep-sided valleys carved by swift-flowing rivers, the most prominent of which is the Meuse. Its most populous cities are Verviers in Belgium and Charleville-Mézières in France, both exceeding 50,000 inhabitants. The Ardennes is otherwise relatively sparsely populated, with few of the towns exceeding 10,000 inhabitants. (Exceptions include Belgium's Eupen and Bastogne.)

The Eifel range in Germany adjoins the Ardennes and is part of the same geological formation, although they are conventionally regarded as being two distinct areas.^{[citation needed]}

The Ardennes are the remnants of a mountain range formed during the Hercynian orogeny; in France similar formations are the Armorican Massif, the Massif Central, and the Vosges. The low interior of such former mountains often contains coal, plus iron, zinc and other metals in the sub-soil. This geologic fact explains the greatest part of the geography of Wallonia and its history. In the North and West of the Ardennes lie the valleys of the Sambre and Meuse rivers, forming an arc (Sillon industriel) going across the most industrial provinces of Wallonia, for example Hainaut Province, along the river Haine (the etymology of Hainaut); the Borinage, the Centre and Charleroi along the river Sambre; Liège Province along the river Meuse.

The region the Ardennes are part of has been uplifted further in the last few hundred thousand years by a mantle plume, as measured from the present elevation of old river terraces, with the largest amount of uplift concentrated in the east, where the Ardennes connect with the Eifel, where the same mantle plume is also responsible for volcanic activity.