Documenting impact has been a continual pursuit in continuing education (CE) for health professionals. Hundreds of primary impact studies have been published, along with 16 impact study syntheses that have sought to generalize about the effectiveness of CE and sometimes how and why the effects occur. This article describes and critiques the methodology of the 16 syntheses and summarizes their findings. A first wave of syntheses established a general causal connection between CE and impacts, but explained impact variability only in the dependent variable-knowledge, competence, performance, or outcome. A second wave added a search for causal explanation through analysis of variables that moderate impact. This wave has begun to identify the most appropriate types of programs for promoting performance changes. The article concludes by suggesting questions and methods for future primary studies and meta-analyses, including improved experimental and meta-analytical methods, along with case and naturalistic studies and action research.