Prior to the construction of the theory of cognitive dissonance, the dominant view in American experimental psychology held that behavior, including verbal attitude statements, was learned and shaped by rewards and/or punishments. Dissonance theory took a surprisingly different view by stating that behaviors and reasons for engaging in them could be strengthened by minimizing the very rewards or threatened punishments that produced the behaviors. Many hundreds of experimental studies later, the main argument of dissonance theory is well supported, although researchers disagree about the necessary and sufficient conditions. The present paper traces the major lines of research, including contributions from around the world, and the major controversies among some of the researchers.