‘Don’t forget the mask’—although most people nowadays follow this advice, professionals express different opinions about the effectiveness of face masks. An international team led by researchers from the Max Planck Institute for Chemistry in Mainz, Germany, has now used observational data and model calculations to answer open questions. The study shows under which conditions and in which way masks actually reduce individual and population-average risks of being infected with COVID-19 and help mitigate the corona pandemic. In most environments and situations, even simple surgical masks effectively reduce the transmission of SARS-CoV-2 and the effective reproduction number for COVID-19. In environments with potentially high airborne virus concentrations such as medical settings and densely occupied indoor spaces, however, masks with higher filtration efficiency (N95/FFP2) should be used and combined with other protective measures such as intensive ventilation.