The Chicago­Joliet­Naperville­Gary, Indiana­Kenosha, Wisconsin Consolidated Metropolitan Statistical Area had 9,157,540 inhabitants during the 2000 census. As of February 2005, there is now an additional classification, that of a "Metropolitan Division." The term Metropolitan Division is used to refer to a county or group of counties within a Metropolitan Statistical Area that has a population core of at least 2.5 million. While a Metropolitan Division is a subdivision of a larger Metropolitan Statistical Area, it often functions as a distinct social, economic, and cultural area within the larger region.

Population estimates for 2003 place the Chicagoland population at approximately 9,650,000, which would make it tied for 25th most populous metropolitan area in the world. Chicagoland runs together with Suburban Milwaukee creating a megalopolis region. Megalopolis (Greek: large city, great city) can mean:
· The city of Megalopolis, Greece.
· Megacity or "megapolis", an extensively large metropolitan area, or a long chain of continuous metropolitan areas.

The suburbs, surrounded by easily annexed flat ground, have been expanding at a tremendous rate since the early 1960s. The 1960s was a turbulent decade of change around the world. Many of the trends of the 1960s were due to the demographic changes brought about by the baby boom generation and the dissolution of European colonial empires. Settlement patterns in Chicagoland tend to follow those in the city proper: the northern suburbs along the shore of Lake Michigan are comparatively affluent, while the southern suburbs are less so, with lower median incomes and a lower cost of living. The southern portion of Chicagoland is occasionally called Illiana. Lake Michigan is one of the five North American Great Lakes. It is bounded, in clockwise order from the south, by the US states of Indiana, Illinois, Wisconsin, and Michigan, which is named for it.
The word Michigan was originally used to refer to the lake itself, and is believed to come from the Chippewa Indian word meicigama, meaning "great water."

Lake Michigan is the only one of the Great Lakes wholly within the borders of the US; the others are shared with Canada. It has a surface area of 22,300 square miles (57750 square km), making it the largest freshwater lake in the US, and the 5th largest lake in the world. It is 307 miles long by 118 miles wide. It greatest depth is 923 feet (281 m). It contains a volume of 4,918 cubic km of water. Its surface averages 580 feet above sea level, the same as Lake Huron, to which it is connected through the Straits of Mackinac. Geologically, Michigan and Huron are the same body of water, but are geographically distinct. They are separated by the Mackinac Bridge. Both lakes are part of the Great Lakes Waterway. In earlier maps of the region, the name "Lake Illinois" has been found in place of "Michigan".