Warren County is a county located in the eastern portion of the U.S. state of Missouri. As of the 2020 census, the population was 35,532. The county is located on the north side of the Missouri River. Its county seat is Warrenton. The county was established on January 5, 1833, and was named for General Joseph Warren, who died in the Battle of Bunker Hill during the American Revolutionary War.

Greater St. Louis is the 21st-largest metropolitan statistical area (MSA) in the United States, the largest in Missouri, and the second-largest in Illinois. Its core city—St. Louis, Missouri—sits in the geographic center of the metro area, on the west bank of the Mississippi River. The river bisects the metro area geographically between Illinois and Missouri, although the latter portion is much more populous. The MSA includes St. Louis County, which is independent of the City of St. Louis; their two populations are generally tabulated separately.