Psychological certainty has been the subject of a great deal of research across a number of different literatures. This review focuses on prior and ongoing research on attitude certainty—the subjective sense of confidence of conviction a person has about an attitude—to provide a general overview of the role of certainty in attitudes and persuasion. First, we describe the antecedents, or origins, of attitude certainty, with particular attention to the metacognitive appraisals that drive people’s feelings of certainty or uncertainty about their own attitudes. Second, we review the known consequences of attitude certainty, emphasizing the role of certainty in shaping information processing, attitude strength, and attitudinal advocacy. Third, we discuss recent developments that point to an upside for uncertainty in persuasion, whereby uncertainty experienced during message processing can increase message engagement and, thus, enhance message impact. Finally, we highlight several promising directions for future research. Our hope is that this review helps organize classic and contemporary research on attitude certainty and, in so doing, sparks new interest and continuing progress in the years to come.