This research introduces the concept of implicit theories of attitude stability. Across three studies, individuals are shown to vary both naturally and situationally in their lay theories about the stability of attitudes. Furthermore, these general theories are shown to impact people’s certainty in their specific attitudes by shaping their perceptions of the stability of the attitude under consideration. By affecting attitude certainty, implicit theories of attitude stability also influence the extent to which people rely on their attitude when committing to future attitude-relevant behavior. Moreover, following exposure to a persuasive attack, implicit theories are shown to interact with situational perceptions of attitude stability to determine attitude certainty. Collectively, these findings suggest that implicit theories of attitude stability have an important influence on people’s attitude certainty, subsequent behavioral intentions, and resistance to persuasive messages. Future directions concerning the potential impact of these theories for other attitudinal phenomena are discussed.