Introduction: This article critiques the questions asked and methods used in research syntheses in continuing education (CE) in the health professions, summarizes the findings of the syntheses, and makes recommendations for future CE research and practice. Methods: We identified 1.5 research syntheses published after 1993 in which primary CE studies were reviewed and the performance (behavior) of health professionals and/or patient health outcomes were examined. Results: The syntheses were categorized by the research questions they asked using a wave metaphor: Wave One (n = 3) syntheses sought to establish a general descriptive causal connection between CE and impact variables, asking, "Is CE effective, and for what outcomes?" Wave Two syntheses (n = 12) sought to explain the relationship between CE and impacts by identifying causal moderators, asking, "What kinds of CE are effective?" Discussion: Wave One findings confirm previous research that CE can improve knowledge, skills, attitudes, behavior, and patient health outcomes. Wave Two syntheses show that CE, which is ongoing, interactive, contextually relevant, and based on needs assessment, can improve knowledge, skills, attitudes, behavior, and health care outcomes. The most important implication of the present review is that there are differential impacts of CE programs, and the reasons for those impacts cannot be fully understood unless the context of the program is considered. (Contains 1 table.)