With the growth of online courses, moderators can play a significant role in engaging and supporting learners in asynchronous online discussions (AODs). However, a synthesis of the research on moderators has not been performed in the 40 years since the term was first used in reference to online learning. We examined 76 studies to determine key concepts, characteristics, and factors that researchers have identified regarding moderation in educational AODs. We found disparate perspectives regarding moderator definitions, identities, roles, duties, and training approaches. Based on a comparative analysis of four conceptual frameworks on moderation, we developed a taxonomy that delineates moderator duties into managerial, monitoring, pedagogical, technical, and social roles. Understanding how moderation can be used effectively in AODs could direct future research and applications to practice by informing the development of supportive resources and training. We conclude by defining protocols applicable to a future systematic review in this area.