Professional development--formal in-service training to upgrade the content knowledge and pedagogical skills of teachers--is widely viewed as an important means of improving teaching and learning. While many interventions "include" professional development, professional development was the central intervention of the two recent research and demonstration projects--the Professional Development in Reading Study (the "Reading PD study," for short) and the Middle School Mathematics Professional Development Impact Study (the "Math PD study")--whose findings are synthesized in this report. This report reviews the findings of the two studies and reflects on their meaning. It offers an important caution to educators and policymakers: Professional development cannot be counted on to improve outcomes for students. In both the studies examined here, the professional development--which went far beyond the "one-shot" approach that has been widely decried--had only limited effects on teachers' knowledge and instruction and did not have an impact on student test scores. (Contains 4 tables, 1 figure and 18 footnotes.)