Findings are reported from a six-year study of seven expert mathematics teachers who taught in elementary schools in very poor neighborhoods with "difficult" students or in economically depressed areas with high unemployment. Compared with novice teachers, the expert teachers used time more wisely, organized lessons better, and knew their content and how to help children learn it. Each of these aspects is discussed with specifics noted. The three aspects are intertwined, and expert teachers know how to combine the three so that their students are successful. (MNS)