Will County is a county located in the northern part of the state of Illinois. According to the 2010 census, it had a population of 677,560, which is an increase of 34.9% from 502,266 in 2000, making it the fourth-most populous county in Illinois. The county seat is Joliet. Will County is one of the five collar counties of the Chicago-Naperville-Elgin, IL-IN-WI Metropolitan Statistical Area. The portion of Will County around Joliet uses the 815 and 779 area codes, 630 and 331 area code for far northern Will County, and 708 area code for eastern Will County. Will County was formed in 1836 out of Cook and Iroquois. It was named after Dr. Conrad Will, a businessman involved in salt production in southern Illinois, and also a politician. Will was a member of the first Illinois Constitutional Convention and a member of the Illinois Legislature until his death in 1835. On January 12, 1836, Will County was formed from Cook County and Iroquois County. It included besides its present area, the part of Kankakee County, Illinois lying north of the Kankakee River. Will County lost that area when Kankakee County was organized in 1852, but since then its boundaries have been unchanged.