Joellen Killion is senior advisor to Learning Forward. In each issue of "The Learning Professional", Killion explores a recent research study to help practitioners understand the impact of particular professional learning practices on student outcomes. In this Issue Mary Kennedy conducts a review and analysis of the research on professional development in K-12 U.S. schools in the core content areas published since 1975. Acknowledging that past reviews of professional development research based on its core features have insufficiently considered the variance in research designs and professional development content and design, Kennedy approaches the review with different theories of action about how professional development influences teacher learning and enactment of learning in practice. The analysis yields a graphical as well as a statistical representation of effects that allows for alternative comparison of studies across contexts and for various types of interpretation.