This article identifies the effects of work-related training on worker productivity by exploiting a field experiment that randomly assigns workers to treatment and control groups combined with data on worker performance before and after training. We find that participation in the training programme leads to a 10% increase in performance. Moreover, we provide experimental evidence for externalities from training: An increase of 10 percentage points in the share of treated peers improves a worker's performance by 0.51%. Furthermore, we find that the performance increase is not due to lower quality provided by the worker.