In-service teacher training aims to improve the supply of public education. A randomized experiment was conducted in Mexico to test whether teacher training could increase teacher efficiency in public secondary schools. After seven and a half months of exposure to a trained teacher, students improved their English. This paper explores two mechanisms through which training can affect student learning. First, trained teachers improved their English by 0.35 standard deviations in the short run. Teachers in the control group caught up with treatment teachers by the end of the school year in part because teachers in the treatment group reduced out-of-pocket expenditures to learn English in 53 percent. Second, teachers changed classroom practices by providing more opportunities for students to actively engage in learning. This evidence suggests that teacher training may be effective at improving student learning and that teacher incentives may play a role in mediating its effects.