Delaware County is a county located in the U.S. state of Pennsylvania. As of the 2010 census, the population was 558,979, making it Pennsylvania's fifth most populous county, behind Philadelphia, Allegheny, Montgomery, and Bucks counties. Its county seat is Media.
Delaware County was created on September 26, 1789, from part of Chester County and named for the Delaware River. 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-county of Philadelphia, and is included in the Delaware Valley area. Socioeconomically, Delaware County consists of mostly middle-class and upper middle class communities, with some upper-class or working class neighborhoods. Delaware County is the only county covered in its entirety by area codes 610 and 484.