Will County is a county in the northeastern part of the state of Illinois. According to the 2020 census, it had a population of 696,355, an increase of 2.8% from 677,560 in 2010, making it Illinois's fourth-most populous county. The county seat is Joliet.

Will County was formed on January 12, 1836, out of Cook and Iroquois Counties. It was named after Conrad Will, a politician and businessman involved in salt production in southern Illinois.[3] Will was a member of the first Illinois Constitutional Convention and a member of the Illinois legislature until his death in 1835. Besides its present area, the county originally included the part of Kankakee County, Illinois, north of the Kankakee River. It lost that area when Kankakee County was organized in 1852. Since then its boundaries have not changed.