Jefferson County is the fourth most populous county of the State of Colorado. The United States Census Bureau estimates that the county population was 526,801 in 2005. The county seat is Golden and the most populous city is Lakewood. The Colorado state center of population is located in Jefferson County.
Located in the Front Range of the Rocky Mountains, northeast Jefferson County is part of the Denver Metropolitan Area. The major expressway Interstate 70 traverses Jefferson County from east-to-west, passing just south of the county seat of Golden. Jefferson County is a quite long county from north to south, following the Front Range.
A major employer in Jefferson County is the large Coors Brewing Company in Golden. Also, the state-supported Colorado School of Mines is located in Jefferson County, offering programs in mining and engineering.
Jefferson County was one of the original 17 counties created by the Territory of Colorado on 1861-11-01. Jefferson County took the name of predecessor Jefferson County of the extralegal Territory of Jefferson. Robert Williamson Steele, Governor of the Provisional Government of the Territory of Jefferson from 1859 to 1861, built his home in the county at Mount Vernon and later at Apex. Jefferson County and the Jefferson Territory were named for Thomas Jefferson, the principal author of the United States Declaration of Independence and the third President of the United States.
In 1908, the southern tip of Jefferson County was transferred to Park County, reducing Jefferson County to its present length of 54 miles (87 kilometers). Several annexations by the City and County of Denver and the 2001 consolidation of the City and County of Broomfield removed eastern portions of the county. The Columbine High School massacre occurred on 1999-04-20, in the Columbine Area of Jefferson County.
Jefferson County is often nicknamed Jeffco. The name Jeffco is incorporated in the official name of the North Jeffco Park and Recreation District, the Jeffco Business Center Metropolitan District No. 1, and several businesses located in Jefferson County. Jeffco is also incorporated in the unofficial monikers of many Jefferson County agencies. The Rocky Mountain Metropolitan Airport operated by Jefferson County was previously known as the Jeffco Airport.
- Arvada (mostly)
- Littleton (partially)
- Westminster (partially)
- Wheat Ridge
- Bow Mar
- Mountain View
- Superior (partially)
Unincorporated Census Designated Places
- Aspen Park
- East Pleasant View
- Indian Hills
- Ken Caryl
- West Pleasant View