E-cigarettes present an opportunity to significantly accelerate already declining smoking rates, and thereby tackle one of the largest causes of death in the UK today. They are substantially less harmful—by around 95%—than conventional cigarettes. They lack the tar and carbon monoxide of conventional cigarettes—the most dangerous components. It has also proven challenging to measure the risks from ‘second-hand’ e-cigarette vapour because it is negligible and substantially less than that of conventional cigarettes. There are uncertainties, nevertheless, especially about any long-term health effects, because the products have not yet had a history of long use. Ultimately, however, any judgement of risks has to take account of the risk of not adopting e-cigarettes—that is, continuing to smoke conventional cigarettes, which are substantially more harmful. Existing smokers should be encouraged to give up, but if that is not possible they should switch to e-cigarettes as a considerably less harmful alternative.