Aerotek Inc., a division of Allegis Group, announced last week that Thomas Thornton will retire as president and CEO. However,, Thornton will join Allegis Group’s board of directors, effective Jan. 1, 2012.

Thornton has been with Aerotek for 21 years, 13 as its president and CEO.

Aerotek ranked No. 1 on Staffing Industry Analysts’ 2011 lists of largest U.S. engineering/design staffing firms and clinical/scientific staffing firms. Allegis Group ranked No. 4 on SIA’s 2011 list of largest staffing firms in the world.