People can clearly incubate and transmit SARS-CoV-2 in the absence of symptoms. Nonetheless, the value of mass testing (large scale asymptomatic screening to identify cases) has been contested. In principle, isolation of people with pre-symptomatic or asymptomatic SARS-CoV-2 infection will prevent further spread. In practice, it is less clear whether enough infectious people can be identified through screening to have a quantitatively important effect on transmission, and whether the direct benefits of enhanced case ascertainment are outweighed by direct or indirect costs. The debate is complicated by an absence of data from randomised controlled trials, and controversy about the suitability of lateral flow tests for this purpose.