William E. Peterson (born February 2, 1936) is an American politician and educator who served as a member of the Illinois Senate from 1993 to 2009. He had also served as the Assistant Minority Leader. Peterson previously served in the Illinois House of Representatives from 1983 to 1993. Peterson had announced that he would not seek reelection in 2008. Republican candidate Dan Duffy of Lake Barrington defeated Bill Gentes, the mayor of Round Lake, in the November 4, 2008 election. Peterson is the longtime Supervisor for Vernon Township in Lake County, Illinois.