The Chicago Metropolitan Statistical Area (MSA) was originally designated by the United States Census Bureau in 1950. It consisted of theIllinois counties of Cook, DuPage, Kane, Lake and Will, along with Lake County in Indiana. As surrounding counties saw an increase in their population densities and the number of their residents employed within Cook County, they met Census criteria to be added to the MSA. The Chicago MSA, now defined as the Chicago-Naperville-Elgin, IL-IN-WI Metropolitan Statistical Area, is the third largest MSA by population in the United States with a population of 10,055,638 (2009 estimate). The Chicago metro area is the largest in an emerging Great Lakes Megalopolis.
The Chicago MSA is further subdivided by state boundaries into the Chicago-Naperville-Joliet, IL Metropolitan Division, corresponding roughly to the CMAP region; the Gary, IN Metropolitan Division consisting of the Indiana counties of Lake and Porter, as well as two surrounding counties; and the Lake County-Kenosha County, IL-WI Metropolitan Division.
The 2012 estimated population total for the MSA is 9,522,434. A breakdown of the 2009 estimated populations of the three Metropolitan Divisions of the MSA are as follows:
Combined Statistical Area 
The OMB also defines a slightly larger region as a Combined Statistical Area (CSA). The Chicago-Naperville, IL-IN-WI Combined Statistical Area combines the metropolitan areas of Chicago, Michigan City (in Indiana), and Kankakee (in Illinois). This area represents the extent of the labor market pool for the entire region. The CSA has a population of 9,899,902 (2012 estimate).
United Nations' Chicago urban agglomeration 
The Chicago urban agglomeration, according to the United Nations World Urbanization Prospects report (2011 revision), lists a population of 9,545,000. The term “urban agglomeration” refers to the population contained within the contours of a contiguous territory inhabited at urban density levels. It usually incorporates the population in a city plus that in the suburban areas.
Chicagoland is an informal name for the Chicago metropolitan area. There is no official definition for Chicagoland, which may be larger than the MSA and include portions of the greater CSA.
Colonel Robert R. McCormick, editor and publisher of the Chicago Tribune, usually gets credit for placing the term in common use. McCormick's conception of Chicagoland stretched all the way to nearby parts of four states (Indiana, Wisconsin, Michigan, and Iowa). The first usage was in theTribune's July 27, 1926 front page headline: "Chicagoland's Shrines: A Tour of Discoveries" by reporter James O'Donnell Bennett. He stated that Chicagoland comprised everything in a 200-mile (320 km) radius in every direction and reported on many different places in the area. The Tribune was the dominant newspaper in a vast area stretching to the west of the city, and that hinterland was closely tied to the metropolis by rail lines and commercial links.
Today, the Chicago Tribune's usage includes the city of Chicago, the rest of Cook County, eight nearby Illinois counties (Lake, McHenry, DuPage,Kane, Kendall, Grundy, Will and Kankakee), and two counties in Indiana, Lake and Porter. Illinois Department of Tourism literature uses Chicagolandfor suburbs in Cook, Lake, DuPage, Kane and Will counties, treating the city separately. The Chicagoland Chamber of Commerce defines it as all of Cook, and DuPage, Kane, Lake, McHenry and Will counties.
However, there are differing viewpoints, which are usually the point of view of whomever one speaks with. For example, many residents who live in the further out satellite counties still refer to themselves as being "from Chicago" or "Chicagoans." Until recently, DeKalb County wasn't considered part of the region, but as the region continues to grow, it is now commonly defined as the "far suburbs" of the city.
In addition, company marketing programs such as Construction Data Company's "Chicago and Vicinity" region and the Chicago Automobile Trade Association's "Chicagoland and Northwest Indiana" advertising campaign are directed at the MSA itself as well as LaSalle, Winnebago (Rockford), Boone, and Ogle counties in Illinois, as well as Jasper, Newton and La Porte counties in Indiana and Kenosha, Racine and Walworth counties in Wisconsin and even as far northeast as Benton Harbor, Michigan. The region is part of the Great Lakes Megalopolis containing an estimated 54 million people.
Chicago Metropolitan Agency for Planning 
Chicago Metropolitan Agency for Planning (CMAP) is an Illinois state agency responsible for transportation infrastructure, land use, and long term economic development planning for the areas under its jurisdiction within Illinois. The planning area has a population of over 8 million, which includes the following locations in Illinois:
- Cook County
- DuPage County
- Kane County
- Kendall County
- Lake County
- McHenry County
- Will County
- Aux Sable Township, Grundy County