Area code 636 is a telephone area code in North American Numbering Plan (NANP) for the east-central part of the U.S. state of Missouri, comprising mainly the western suburbs of St. Louis. It includes parts of Chesterfield and Fenton in St. Louis County, and all of St. Charles, Jefferson, and Warren counties, all of Franklin County with the exception of Sullivan, as well as the towns of High Hill and Jonesburg in Montgomery County and the southern portion of Lincoln County. It was created in 1999 in an area code split from area code 314, which was one of the original North American area codes of 1947.