Recent research in Tennessee and Texas (see e.g., Haycock, 1998) focuses our attention on the teacher as the key to student learning. Although we have a fairly substantial knowledge of what constitutes teacher effectiveness for certain kinds of outcomes and certain groups of students (see e.g., Good & Brophy, 2000), we have not yet determined how best to increase the effectiveness of teachers. The challenge intensifies in settings populated by teachers who may have been prepared for very different kinds of children, families, and classrooms than they encounter in classrooms today. Because quality of teaching makes a difference in student performance (Haycock, 2003), this chapter focuses on teacher professional development that makes a difference in the performance of students from traditionally underrepresented populations, particularly those from diverse cultural and linguistic groups. More specifically, the chapter reviews the status of research on professional development for teachers of diverse students and summarizes selected findings from a recent synthesis of research on that topic (see Knight & Wiseman, in press).