Monroe County is a county located in the U.S. state of Illinois. According to the 2020 census, it had a population of 34,962. Its county seat and largest city is Waterloo.

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.