Chester County, colloquially referred to as Chesco, is a county in the Commonwealth of Pennsylvania. It is located in the Delaware Valley region, located in the southeastern part of the state. As of the 2020 census, the population was 545,823(As of 2022), increasing by 7.1% from 498,886 in 2010. The county seat is the Borough of West Chester. The most populous of its 73 municipalities is Tredyffrin Township. The most populous boroughs are West Chester and Phoenixville. Coatesville is the only municipality in the County organized under the City form of government, a technical rather than demographic distinction. Chester County was one of the three original Pennsylvania counties created by William Penn in 1682. It was named for Chester, England. The county is part of the Southeast Pennsylvania region of the state.

Pennsylvania, officially the Commonwealth of Pennsylvania, is a state spanning the Mid-Atlantic, Northeastern, Appalachian, and Great Lakes regions of the United States. Pennsylvania borders Delaware to its southeast, Maryland to its south, West Virginia to its southwest, Ohio and the Ohio River to its west, Lake Erie and New York to its north, the Delaware River and New Jersey to its east, and the Canadian province of Ontario to its northwest.