This research explores the effect of source certainty—that is, the level of certainty expressed by a message source—on persuasion. The authors propose an incongruity hypothesis, suggesting that source certainty effects depend on perceived source expertise. In three experiments, consumers receive persuasive messages from sources of varying expertise and certainty. Across studies, low expertise sources violate expectancies, stimulate involvement, and promote persuasion when they express certainty, whereas high expertise sources violate expectancies, stimulate involvement, and promote persuasion when they express uncertainty. Thus, nonexpert (expert) sources can gain interest and influence by expressing certainty (uncertainty).