Chester County is a county located in the U.S. state of Pennsylvania. As of 2000, the population was 433,501. The county seat is West Chester. It is the highest-income county in Pennsylvania and 24th highest in the nation as measured by median household income as of 2010. Chester County is home to many communities that comprise the westernmost extent of the wealthy Main Line suburbs. According to the U.S. Census Bureau, the county has a total area of 760 square miles, of which, 756 square miles of it is land and 4 square miles of it (0.51%) is water. The topography consists of rolling hills and valleys and it is part of the region known as the Piedmont.
Chester County is one of the three original counties of Pennsylvania created by William Penn in 1682. It is named for Cheshire, England. It is part of the Delaware Valley area, and is the only Delaware Valley county in Pennsylvania that does not border Philadelphia, though it is close.
Chester, Philadelphia, and Bucks were the three original Pennsylvania counties created by William Penn on August 24, 1682. At that time, Chester County's borders were Philadelphia County to the north, the western edge of the colony (approximately the Susquehanna River) to the west, the Delaware River to the east, and Delaware and Maryland to the south. Chester County replaced the Pennsylvania portion of New Netherland/New York’s Upland, which was officially eliminated when Pennsylvania was chartered on March 4, 1681, but did not actually cease to exist until June of that year. Much of the Welsh Tract was in eastern Chester County and Welsh place names continue to predominate there.
The fourth county in the state, Lancaster County, was formed from Chester County on May 10, 1729. On March 11, 1752, Berks County was formed from the northern section of Chester County, as well as parts of Lancaster and Philadelphia Counties.
The original Chester County seat was the naval shipbuilding city of Chester. However, it became part of Delaware County when that county was formed from the eastern portion of the Chester County on September 26, 1789. This took the county seat out of Chester County, so West Chester became the new county seat that year, and has remained so to the present.
Much of the history of Chester County arises from its location between Philadelphia and the Susquehanna River. The first road to "the West" (meaning Lancaster County) passed through the central part of Chester County; with some re-alignments, it became the Lincoln Highway and later U.S. Route 30. This road is still named Lancaster Avenue in most of the Chester county towns it runs through. The first railroad (which became the Pennsylvania Railroad) followed much the same route, and the Reading Railroad progressed up the Schuylkill River to Reading. Industry tended to concentrate along the rail lines. In time, easy transportation allowed workers to commute to urban jobs, and the rise of the suburbs followed. To this day, the built-up areas form "fingers" extending along lines of transportation.
The Battle of Brandywine was fought at what is now the southeastern fringe of the county, and the Valley Forge encampment was at the northeastern edge. The former Valley Forge Army Hospital, constructed during World War II, was one of the largest military hospitals in the United States.