Chicago is the third largest city in the United States, and with more than 2.8 million people, the largest city in the state of Illinois. Located on the southwestern shores of Lake Michigan and next to Indiana, Chicago is the third-most densely populated major city in the U.S., and anchor to the world's 26th largest metropolitan area with over 9.6 million people across three states. Except for the southwest corner of O'Hare Airport in DuPage County, the city of Chicago is located in Cook County.

Chicago was founded in 1833, near a portage between the Great Lakes and the Mississippi River watershed. The city became a major transportation and telecommunications hub in North America. Today, the city retains its status as a major hub, both for industry and infrastructure, with its O'Hare International Airport as the second busiest airport in the world. In 2007, the city attracted 32.8 million domestic visitors and about 1.15 million foreign visitors.

In modern times, the city has taken on an additional dimension as a center for business and finance and is listed as one of the world's top ten Global Financial Centers. Chicago is a stronghold of the Democratic Party and has been home to influential politicians, including the current President of the United States, Barack Obama. The World Cities Study Group at Loughborough University rated Chicago as an, "alpha world city."

Globally recognized, Chicago has numerous nicknames, which reflect the impressions and opinions about historical and contemporary Chicago. The best known include: "Chi-town"; "Chi-city"; the "Windy City" with reference to Chicago politicians and residents boasting about their city; "Second City," due to the city generally being the second most prestigious in the nation in terms of culture, entertainment, and finance; and because for much of the twentieth century Chicago's population was the second largest of any city in the United States, and the "City of Big Shoulders", referring to its numerous skyscrapers (whose steel frame designs were largely pioneered in Chicago), described as being husky and brawling. Chicago has also been called "the most American of big cities".

The 1920s brought notoriety to Chicago as gangsters, including the notorious Al Capone, battled each other and law enforcement on the city streets during the Prohibition era. Chicago had over 1,000 gangs in the 1920s. The 1920s also saw a major expansion in industry. The availability of jobs attracted African Americans from the South. Arriving in the tens of thousands during the Great Migration, the newcomers had an immense cultural impact. It was during this wave that Chicago became a center for jazz, with King Oliver leading the way. In 1933, Chicago Mayor Anton Cermak was fatally wounded in Miami during a failed assassination attempt on President Franklin D. Roosevelt.

.On December 2, 1942, physicist Enrico Fermi conducted the world's first controlled nuclear reaction at the University of Chicago as part of the top-secret Manhattan Project.

Mayor Richard J. Daley was elected in 1955, in the era of machine politics. Starting in the 1960s, many residents, as in most American cities, left the city for the suburbs. Structural changes in industry caused heavy losses of jobs for lower skilled workers. In 1966 James Bevel, Martin Luther King, Jr., and Albert Raby led the Chicago Open Housing Movement, which culminated in agreements between Mayor Richard J. Daley and the movement leaders. Two years later, the city hosted the tumultuous 1968 Democratic National Convention, which featured physical confrontations both inside and outside the convention hall, including full-scale riots, or in some cases police riots, in city streets. Major construction projects, including Sears Tower (which in 1974 became the world's tallest building), University of Illinois at Chicago, McCormick Place, and O'Hare Airport, were undertaken during Richard J. Daley's tenure. When he died, Michael Anthony Bilandic was mayor for three years. His loss in a primary election has been attributed to the city's inability to properly plow city streets during a heavy snowstorm. In 1979, Jane Byrne, the city's first female mayor, was elected. She popularized the city as a movie location and tourist destination.

In 1983 Harold Washington became the first African American to be elected to the office of mayor, in one of the closest mayoral elections in Chicago. After Washington won the Democratic primary, racial motivations caused a few Democratic alderman and ward committeemen to back the Republican candidate Bernard Epton, who ran on the slogan Before it's too late, a thinly veiled appeal to fear. Washington's term in office saw new attention given to poor and minority neighborhoods. His administration reduced the longtime dominance of city contracts and employment by ethnic whites. Washington died in office of a heart attack in 1987, shortly after being elected to a second term. Current mayor Richard M. Daley, son of Richard J. Daley, was elected in 1989. He has led many progressive changes to the city, including improving parks; creating incentives for sustainable development, including green roofs; and major new developments. Since the 1990s, some neighborhoods have undergone revitalization in which some lower class areas have been transformed to high priced and middle-class neighborhoods.

In 2003, Meigs Field, an airport close to downtown, was demolished without advance warning by the order of mayor Richard Daley, who wanted the land for redevelopment. Private aircraft using the airport were stranded when the runway was destroyed. They were later permitted to depart from a taxiway.

Chicago is located in northeastern Illinois at the southwestern tip of Lake Michigan. It sits on a continental divide at the site of the Chicago Portage, connecting the Mississippi River and the Great Lakes watersheds. The city lies beside Lake Michigan, and two rivers—the Chicago River in downtown and the Calumet River in the industrial far South Side—flow entirely or partially through Chicago. The Chicago Sanitary and Ship Canal connects the Chicago River with the Des Plaines River, which runs to the west of the city. Chicago's history and economy are closely tied to its proximity to Lake Michigan. While the Chicago River historically handled much of the region's waterborne cargo, today's huge lake freighters use the city's Lake Calumet Harbor on the South Side. The lake also provides another positive effect, moderating Chicago's climate; making waterfront neighborhoods slightly warmer in winter and cooler in summer.

When Chicago was founded in the 1830s, most of the early building began around the mouth of the Chicago River, as can be seen on a map of the city's original 58 blocks. The overall grade of the city's central, built-up areas, is relatively consistent with the natural flatness of its overall natural geography, generally exhibiting only slight differentiation otherwise. The average land elevation is 579 ft (176 m) above sea level. The lowest points are along the lake shore at 577 ft (176 m), while the highest point, at 735 ft (224 m), is a landfill located in the Hegewisch community area on the city's far south side.

Lake Shore Drive runs adjacent to a large portion of Chicago's lakefront. Parks along the lakeshore include: Lincoln Park, Grant Park, Burnham Park and Jackson Park; 29 public beaches are also found along the shore. Near downtown, landfills extend into the Lake, providing space for the Jardine Water Purification Plant, Navy Pier, Northerly Island, the Museum Campus, Soldier Field and large portions of the McCormick Place Convention Center. Most of the city's high-rise commercial and residential buildings can be found within a few blocks of the lake.

Chicagoland is an informal name for the Chicago metro area, used primarily by copywriters, advertising agencies, and traffic reporters. There is no precise definition for the term "Chicagoland", but it generally means the city and its suburbs together. The Chicago Tribune, which coined the term, includes the city of Chicago, the rest of Cook County, eight nearby Illinois counties: Lake, McHenry, DuPage, Kane, Kendall, Grundy, Will and Kankakee, and three counties in Indiana: Lake, Porter, and LaPorte. The Illinois Department of Tourism defines Chicagoland as Cook County without the city of Chicago, and only Lake, DuPage, Kane and Will counties. The Chicagoland Chamber of Commerce defines it as all of Cook and DuPage, Kane, Lake, McHenry and Will counties.