Delaware County (Delco) was created on September 26, 1789 from part of Chester County and named for the Delaware River. Its county seat, since 1851, is Media. Chester City, prior to 1851, was the county seat of both Delaware County and, before that, of Chester County. Socioeconomically, Delaware County runs the gamut from the towns of Chester and Darby to the posh suburban neighborhoods of Radnor and Newtown along the wealthy Main Line to the north. This county is part of the Delaware Valley area and borders Philadelphia.

As of 2000, there were 550,864 people, 206,320 households, and 139,472 families residing in the county. The population density was 1,155/km² (2,990/mi²). There were 216,978 housing units at an average density of 455/km² (1,178/mi²). The racial makeup of the county was 80.32% White, 14.52% African American, 0.11% Native American, 3.29% Asian, 0.02% Pacific Islander, 0.56% from other races, and 1.19% from two or more races. 1.52% of the population were Hispanic or Latino of any race.There were 206,320 households out of which 31.50% had children under the age of 18 living with them, 50.80% were married couples living together, 12.90% had a female householder with no husband present, and 32.40% were non-families. 27.60% of all households were made up of individuals and 11.60% had someone living alone who was 65 years of age or older. The average household size was 2.56 and the average family size was 3.17. In the county, the population was spread out with 24.80% under the age of 18, 8.90% from 18 to 24, 28.80% from 25 to 44, 21.90% from 45 to 64, and 15.60% who were 65 years of age or older. The median age was 37 years. For every 100 females there were 91.20 males. For every 100 females age 18 and over, there were 86.90 males. The median income for a household in the county was $50,092, and the median income for a family was $61,590. Males had a median income of $44,155 versus $31,831 for females. The per capita income for the county was $25,040. About 5.80% of families and 8.00% of the population were below the poverty line, including 10.00% of those under age 18 and 7.10% of those age 65 or over.