Despite massive investments in teacher professional development (PD) programs in developing countries, there is little evidence on their effectiveness. We present the results of a large-scale, randomized evaluation of a high-profile PD program in China, in which teachers were randomized to receive PD; PD plus follow-up; PD plus evaluation of their command of the PD content; or no PD. Precise estimates indicate that PD and associated interventions failed to improve teacher and student outcomes. A detailed analysis of the causal chain shows teachers find PD content to be overly theoretical, and PD delivery too rote and passive, to be useful.