Douglas County is the eighth most populous of the 64 counties of the state of Colorado. The county is located midway between Colorado's two largest cities: Denver and Colorado Springs. The United States Census Bureau that the county population was 285,465 in 2010 census, a 62.4% increase since the 2000 census, making Douglas County one of the fastest growing counties in the United States. Douglas County is part of the Denver-Aurora Metropolitan Statistical Area and the Denver-Aurora-Boulder Combined Statistical Area. The county seat is Castle Rock.
Douglas County has the highest median household income of any Colorado county or statistical equivalentin 2000. In 2008, it ranked #8 in the United States in that category - it was one of two in the top 15 not in the vicinity of New York or Washington.

Douglas County was one of the original 17 counties created in the Colorado Territory by the Colorado Territorial Legislature on November 1, 1861. The county was named in honor of U.S. Senator Stephen A. Douglas of Illinois, who died five months before the county was created. The county seat was originally Franktown, but was moved to California Ranch in 1863, and then to Castle Rock in 1874. Although the county's boundaries originally extended eastward to the Kansas state border, in 1874 most of the eastern portion of the county became part of Elbert County.

Cities: Aurora (partial)· Castle Pines · Littleton (partial) · Lone Tree

Towns: Castle Rock · Larkspur · Parker

CDPs: Acres Green · Castle Pines · Franktown · Grand View Estates · Highlands Ranch · Louviers · Meridian · Perry Park · Roxborough Park · Sedalia · Stonegate · The Pinery · Westcreek

Unincorporated communities: Acequia · Castle Pines Village · Cottonwood · Dakan · Deckers · Greenland · Perry Park Ranch