St. Charles County is in the central eastern part of the U.S. state of Missouri.
As of the 2010 census, the population was 360,485, making it Missouri's third-most populous county.
St. Charles County is part of the St. Louis, MO-IL Metropolitan Statistical Area and contains many of the city's northwestern suburbs.
The wealthiest county in Missouri, St. Charles County is one of the nation's fastest-growing counties. The county is also recognized as very conservative, ranking in the top 100 nationally.
St. Charles County includes an area of vineyards and wineries whose distinction has been nationally recognized. On its rural outer edge along the south-facing bluffs above the Missouri River is an area of numerous wineries, so that Missouri Route 94 is sometimes called the Missouri Weinstrasse. The area includes the Augusta AVA, designated in 1980 as the first American Viticultural Area by the federal government.