The U.S. state of Illinois is divided into ten distinct geographic numbering plan areas in the North American Numbering Plan (NANP), which are assigned a total of seventeen area codes. When the 86 original North American area codes where established in 1947, all of the Chicago metropolitan area was located in numbering plan area 312, the rest of northern Illinois was 815, central Illinois was 217, and southern Illinois was 618.
In 1954, the 217 and 618 numbering plan areas were slightly remapped, and in 1957, area code 815 was split for the assignment of area code 309 to western central Illinois. No further changes were made for 33 years. Since 2023, all numbering plan areas of the state are overlay complexes, requiring ten-digit dialing statewide.