Lake Forest is a city in Lake County, Illinois, United States. The population was 20,059 at the 2000 census. The city is south of Waukegan, Illinois, on the shore of Lake Michigan, and is a part of the Chicago metropolitan area and the North Shore.

Lake Forest was founded around its college and laid out as a town in 1857 as a stop for travelers making their way south to Chicago. The headquarters of Tenneco, Brunswick (both Fortune 500 companies), and BFG Technologies are located in unincorporated Lake Forest. The Chicago Bears training facility and headquarters, Halas Hall, opened in 1997 in west Lake Forest, and the Chicago Fire now train at the Bears' previous facility located on the campus of Lake Forest College. Robert Redford filmed Ordinary People, the story of a Lake Forest family, on location in 1980, and the movie went on to win Best Picture at the Academy Awards. Actor Vince Vaughn graduated from Lake Forest High School, as did author Dave Eggers and musician Andrew Bird.