Using tests to compare nations, states, school districts, schools, teachers, and students has increasingly become a basis for educational reform around the globe. Although tests can be informative, "high-stakes testing" (HST) is an approach to reform that applies rewards and sanctions contingent on test outcomes. Results of HST reforms indicate a plethora of unintended negative consequences, leading some to suggest that HST corrupts educational practices in schools. Although there are many accounts of these negative results, SDT supplies the only systematic theory of motivation that explains these effects. In what follows we describe the motivational principles underlying the undermining effects of HST on teachers and learners alike.