Illinois was a state in the United States. Chicago, the most populous city in the state, was home to the Chicago Bears football team and the Chicago Cubs baseball team.
Doctor Emmett Brown and Marty McFly traveled to Chicago in the year 1927, at the height of organized crime. While there, they helped Marty's cousin, Jim McFly, get out of the Benedict mob.
At some point prior to 1986, according to Biff Tannen, Ellen Baines, the youngest member of the Baines family, moved from Hill Valley, California to Chicago, Illinois.
In 1993, Doc Brown traveled to March 14, 1933 to visit Albert Einstein during his visit to Chicago. He hung a photograph of himself on the train with Albert Einstein in his chicken restaurant.