Lake County is situated in the northeastern corner of the U.S. state of Illinois, along the shores of Lake Michigan. As of the 2020 census, it has a population of 714,342, making it the third-most populous county in Illinois. Its county seat is Waukegan, the tenth-largest city in Illinois. The county is primarily suburban, with some urban areas and some rural areas. Due to its location, immediately north of Cook County, Lake County is one of the collar counties of the Chicago metropolitan area. Its northern boundary is the Wisconsin state line.

Waukegan is the most populous city in and the county seat of Lake County, Illinois, United States. It is an industrial suburb about 36 miles (58 km) north of Chicago. As of the 2020 census, its population was 89,321, making it the seventh-most populous city within the Chicago metropolitan area and the 10th-most populous city in Illinois.