All of New Castle County

New Castle County is the northernmost of the three counties of the U.S. state of Delaware. As of the 2010 census, the population was 538,479, making it the most populous county in Delaware. The county seat is Wilmington.

New Castle County is included in the Philadelphia-Camden-Wilmington, PA-NJ-DE-MD Metropolitan Statistical Area. The county is named after the English city of Newcastle.

New Castle County has the highest population and population density of any Delaware county, even though it is the smallest county in the state by area. It has more people than the other two counties, Kent and Sussex, combined. It is also the most economically developed of the three.