In this paper, we present a review of the field of professional development for school leaders. The paper sets out a framework for defining what professional development is, articulates criteria to define "high quality" professional development, and describes goals for professional development. It then critiques the research on professional development for school leaders in three key areas: (a) conceptualization regarding how professional development can influence practitioners' practice and expertise, (b) instruments (instrumentation) to measure proximate and ultimate effects of professional development, and (c) rigor and scope of research designs. Finally, it suggests an approach and agenda to develop the knowledge base regarding effective professional development for school leaders.