Fifty years ago, Leon Festinger introduced a new concept into the language of social psychology. In his A Theory of Cognitive Dissonance, Festinger (1957) explained that two elements of knowledge that are discrepant with each other could cause a state of arousal that he called cognitive dissonance. Dissonance, experienced as an unpleasant and uncomfortable tension state akin to a drive, needs to be reduced. An individual who is in such a state is motivated to add, subtract, or otherwise alter his or her cognitions in order to be rid of the unpleasant feeling of dissonance. From these basic propositions, the theory has spawned controversy, support, and an ample share of criticism. Over the decades, the culmination of thousands of empirical studies and theoretical refinements is testimony to the impact and significance of cognitive dissonance on individual agents and actors.