Appalachian Mountains, mountain system of E North America, extending in a broad belt c.1,600 mi (2,570 km) SW from the Gaspé Peninsula in Quebec prov., Canada, to the Gulf coastal plain in Alabama. Main sections in the system are the White Mts., Green Mts., Berkshire Hills, Catskill Mts., Allegheny Plateau, Black Mts., Great Smoky Mts., Blue Ridge, and Cumberland Plateau. To the E is the Piedmont (foothill) region. The Appalachians, much-eroded remnants of a great mass formed by folding (see mountains), consist largely of sedimentary rocks. Mt. Mitchell (6,684 ft/2,037 m) in North Carolina's Black Mts. is the highest peak.

The Great Appalachian Valley is a chain of lowlands extending S and W from the Hudson Valley; its main segments are the Lehigh, Lebanon, Cumberland, and Shenandoah valleys; the Valley of Virginia; and the Valley of East Tennessee. Long a major north-south travel and settlement corridor, the Great Valley is one of the most fertile areas in the E United States.

The Appalachians are rich in coal; other resources include iron, petroleum, and natural gas. The scenic ranges also abound in resorts and recreation areas, including Shenandoah and Great Smoky Mts. national parks. The Appalachian Trail winds 2,050 mi (3,299 km) along the ridges of the Appalachians between Mt. Katahdin, Maine, and Springer Mt., Georgia.

Crossed by few passes, the Appalachians were a barrier to early westward expansion and played an important role in U.S. history; major east-west routes like the Cumberland Gap and Mohawk Trail followed river valleys or mountain notches. Appalachia is a name applied to parts of the region that were long characterized by marginal economy, isolation of its people from the U.S. mainstream, and distinctive folkways.