South Mountain (Eastern Pennsylvania) in the context of "Lehigh University"

40°42′N 75°30′W / 40.700°N 75.500°W / 40.700; -75.500

The Lehigh Valley (/ˈliːhaɪ/) is a geographic and metropolitan region formed by the Lehigh River in Lehigh and Northampton counties in eastern Pennsylvania. It is a component valley of the Great Appalachian Valley bounded to its north by Blue Mountain, to its south by South Mountain, to its west by Lebanon Valley, and to its east by the Delaware River and Warren County, New Jersey. The Lehigh Valley is about 40 miles (64 km) long and 20 miles (32 km) wide. The Lehigh Valley's largest city is Allentown, the third-largest city in Pennsylvania and the county seat of Lehigh County, with a population of 125,845 residents as of the 2020 census.

The Lebanon Valley is a geographic region that lies between South Mountain and the Ridge and Valley Province of eastern Pennsylvania. The valley lies almost entirely within Lebanon and Berks counties in Pennsylvania. Portions of the valley lie in eastern Dauphin and northern Lancaster counties in Pennsylvania. It is bound to its southwest by the Susquehanna River and to its northeast by the adjoining Lehigh Valley.

The Lebanon Valley is part of the much longer Great Appalachian Valley, also called the Great Valley, a natural lowland route running northeast–southwest and lying just inland from the Blue Ridge Mountains and South Mountain. The Great Valley, including the Lebanon Valley, has historically been admired for its fertile agricultural land. Beyond the southwestern end of the Lebanon Valley, the Great Valley is known locally as the Cumberland Valley. To the northeast the Great Valley is known locally as the Lehigh Valley. Lebanon Valley College is named for the region. The western part of the valley lies within the Chesapeake Bay watershed. The eastern part lies within the Delaware River watershed.