This publication reviews what is known about professional development in elementary and secondary education including its organization, costs, effects on practice, and principles for guiding future professional development programs. Recent reform efforts that have put higher standards in place are examined for their effect on the practice of teaching. A discussion of the cost of professional development notes that most states and districts do not maintain data on how much they are spending on teacher development. Also identified are promising approaches which respect the expertise of accomplished teachers, are integrated with teachers' work, and are based on current research on teaching and learning. These include teacher networks, collaborations between schools and colleges, professional development schools, national board certification, and teachers as researchers. A discussion of implications for policymakers argues that deliberation should be focused on central issues and set clear goals for policy. This section proposes 13 specific goals. A final section reviews the challenges of professional development improvement to significantly affect the 2.4 million teachers working in the United States. (JB)