Chicago-Naperville-Joliet, IL-IN-WI Metropolitan Statistical Area. Principal Cities: Chicago, IL; Naperville, IL; Joliet, IL; Elgin, IL; Gary, IN; Evanston, IL; Arlington Heights, IL; Schaumburg, IL; Skokie, IL; Des Plaines, IL; Hoffman Estates, IL. Counties: Cook County, DeKalb County, DuPage County, Grundy County, Kane County, Kendall County, McHenry County, Will County, Jasper County, Lake County, Newton County, Porter County, Lake County, IL; Kenosha County, WI