Delaware County is a located in the U.S. state of Pennsylvania. As of 2010, the population was 558,979, making it Pennsylvania’s fifth most populous county, behind Philadelphia, Allegheny, Montgomery, and Bucks counties.

Delaware County was created on September 26, 1789, from part of Chester County and named for the Delaware River. Its county seat, since 1851, is Media[1]. Chester City, prior to 1851, was the county seat of both Delaware County and, before that, of Chester County.

Delaware County consists of communities adjacent to the city of Philadelphia. Socioeconomically, Delaware County consists of working class, middle-class, and upper-class neighborhoods.

2,600 acres (11 km2) of the county are occupied by the Ridley Creek State Park.  The county, immediately adjacent to and west of Philadelphia, is part of the Delaware Valley area.