Alps , great mountain system of S central Europe, c.500 mi (800 km) long and c.100 mi (160 km) wide, curving in a great arc from the Riviera coast on the Mediterranean Sea, along the borders of N Italy and adjacent regions of SE France, Switzerland, SW Germany, and Austria, and into Slovenia. Crystalline rocks, which are exposed in the higher central regions, are the rocks forming Mont Blanc, the Matterhorn, and high peaks in the Pennine Alps and Hohe Tauern; limestone and other sedimentary rocks are predominant (but not continuously present) in the generally lower ranges to the north and south. Mont Blanc (15,771 ft/4,807 m) is the highest peak.

The Alps are divided by rivers and other topographic features into more than 40 subunits for which local names are commonly used. Well-known groups in the W Alps (from the Riviera to the Great St. Bernard Pass) include the Maritime, Ligurian, Cottian, and Graian alps, the Mont Blanc group, and Valle d'Aosta. The highest western peaks are Mont Blanc, Mont Pelvoux, Monte Viso, and the Gran Paradiso; the chief routes across this section are via the Mont Cénis Tunnel and the Great and Little St. Bernard passes. The Central Alps (between the Great St. Bernard and Brenner passes) include, in the south, the Pennine, Lepontine, Phaetian, and Ötztal alps; and, in the north, the Bernina, Glarus, Allgäu, and Bavarian alps. The principal peaks of the Central Alps are Monte Rosa, the Matterhorn, the Finsteraarhorn, the Jungfrau, and the Wildspitze; the chief routes are the Simplon Tunnel and the St. Gotthard, Grimsel, Furka, Splügen, Bernina, and Brenner passes. The E Alps comprise, in the south, the Dolomites, the Carnic Alps, and the Julian Alps; and, in the north, the Hohe Tauern and Niedere Tauern; the principal eastern peak is Grossglockner. Most major routes across the E Alps follow the Brenner and Semmering passes.