For more recent work than that mentioned by Alain, check out this [modern classic by Breuillard, Green, Guralnick, Tao][1]


  [1]: http://Expansion%20in%20finite%20simple%20groups%20of%20Lie%20type%20%20Emmanuel%20Breuillard,%20Ben%20Green,%20Robert%20Guralnick,%20Terence%20Tao