Illinois was admitted as a state to the United States on Dec. 3, 1818. Prior to achieving statehood, it was called the Illinois Territory, and it was founded Feb. 3, 1809. Upon its admission to the Union, Illinois became the 21st state.
Illinois first applied for statehood with the U.S. Congress on Jan. 23, 1818. Between application and admission, the borders changed slightly to cover additional territory along the Lake Michigan shore and the Chicago River. Before it became a territory or a state, Illinois was part of a larger area called the Northwest Territory, which encompassed much of current-day Michigan, Wisconsin, Indiana and Ohio.