This paper analyses the effects of work-related training on workers' productivity. To identify causal effects from training, we combine a field experiment that randomly assigned workers to treatment and control groups with panel data on individual worker productivity before and after training. We find that participation in the training programme leads to a 9 percent increase in productivity. Moreover, we provide evidence for externalities from treated workers on their untreated peers: An increase of 10 percentage points in the share of treated peers leads to a productivity increase of 0.45 percent. We provide evidence that the estimated effects are causal and not due to selective labour turnover.