Chicago is in Cook County, Illinois. The Illinois General Assembly created the county on Jan. 15, 1831. The county seat was what was then Fort Dearborn. It would later become Chicago. Cook County was named after Congressman Daniel Cook.
Cook County is 1,635 square miles in area, which makes it the sixth largest county in Illinois. As of 2013, its estimated population was 5,240,700. This makes it the largest county in population in the state and the second largest county in population in the United States.
Cook County encompasses many cities, townships and villages, including Prospect Heights, Des Plaines, Midlothian, Oak Park, Bedford Park, Lemont Township and Bremen Township.