We analyze data from a field experiment in which an auto-repair firm provided checklists to mechanics and monitored their use. Revenue was 20 percent higher during the experiment, and the effect is equivalent to that of a 1.6 percentage point (10 percent) commission increase. Checklists appear to boost productivity by serving both as a memory aid and a monitoring technology. Despite the large benefits to the firm, mechanics did not use checklists without the firm directly monitoring their use. We show that a moral hazard can explain why mechanics do not otherwise adopt checklists.