The science supports the fact that face coverings save lives. Studies suggest that they cut down the chances of both transmitting and catching the coronavirus, and might reduce the severity of an infection by lessening the dose of the virus. But more-definitive answers about how well masks work or when to use them get complicated: biology, epidemiology, physics and human behaviour all factor in. Even the question of what kinds of study would provide conclusive proof is hard to answer. Nature takes a comprehensive look at the evidence available and the large, randomly controlled trials seeking answers.