Chester County is a county located in the U.S. state of Pennsylvania. As of 2011, the population was 500,182. 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). Eastern Chester County is home to many communities that comprise the Main Line western suburbs of Philadelphia, while part of its southernmost portion is considered suburban Wilmington, Delaware, along with southwest Delaware County.

Chester County was one of the three original Pennsylvania counties created by William Penn in 1682. It was named for Chester, England. It is part of the Delaware Valley region and is the only Delaware Valley county in Pennsylvania that does not actually border Philadelphia.

Philadelphia, Bucks, and Chester were the three Pennsylvania counties initially 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.

Coordinates: 53°11′33″N 2°53′30″W / 53.1926°N 2.8918°W / 53.1926; -2.8918

Chester ( /ˈtʃɛstər/ CHESS-tər), is a city in Cheshire, England. Lying on the River Dee, close to the border with Wales, it is home to 80,121 inhabitants, and is the largest and most populous settlement of the wider unitary authority area of Cheshire West and Chester, which had a population of 328,100 according to the 2001 Census. Chester was granted city status in 1541.

Chester was founded as a "castrum" or Roman fort with the name Deva Victrix in the year 79 by the Roman Legio II Adiutrix during the reign of the Emperor Vespasian. Chester's four main roads, Eastgate, Northgate, Watergate and Bridge, follow routes laid out at this time – almost 2,000 years ago. One of the three main Roman army bases, Deva later became a major settlement in the Roman province of Britannia. After the Romans left in the 5th century, the Saxons fortified the town against the Danes and gave Chester its name. The patron saint of Chester, Werburgh, is buried in Chester Cathedral.