This systematic review examined research on moderators in asynchronous online discussions (AODs) through a review of 52 sources published over the past four decades. Areas of interest included conceptual frameworks cited in research, publication trends, instructional contexts, research methods and characteristics, and descriptions of the role of the moderator with implications for practice. Results indicate: (1) nearly half of the publications did not cite a conceptual framework focused on moderation; (2) the field is diverse with a wide variety of themes for research designs, outcomes, foci, and questions; (3) half of reviewed publications involved case studies or similarly limited study designs; (4) the majority of publications collected data on students in higher education, but there was a lack of consistency in the reporting of demographic information; (5) research foci tended toward investigating peer moderators or the role of the instructor; (6) research questions tended to focus on strategies of moderators or student performance and discussion quality; (7) most definitions or expectations of a moderator included discussion and social management duties. We conclude by discussing the implications of some of the findings and future research options.