As investments in K-12 math and science professional development programs expanded over the past decade, researchers and policymakers have questioned whether and how such programs work to improve student learning. This article summarizes the current knowledge-base and offers a theoretical framework researchers can use to design studies that explore mechanisms through which professional development programs influence teacher knowledge, teacher practices, and ultimately student achievement. We quantitatively pool the most rigorous evaluation research available to determine whether currently held beliefs in the field are supported by the evidence. Although this study does find some support for the guidance propounded by experts, it cautions policymakers and practitioners that the current evidence base is thin. We urge researchers to build on what we have learned through theory and practice to increase the rigor of evaluations on this important topic by incorporating valid and reliable measures of professional development components as well as valid measures of teacher knowledge and practices. (Contains 2 figures, 4 tables and 10 footnotes.)