We examine the effect of obliging employers to enrol employees automatically into a workplace pension scheme. We exploit the phased roll-out of automatic enrolment, by employer size, in the first country to do so nationwide (the UK), to estimate its effect on pension saving among private sector employees. We find substantial increases in pension participation and a rise in pension saving. Surprisingly, many newly enrolled employees received an employer contribution substantially above the (very low) minimum default level. Automatic enrolment also caused the pension participation of those employees who were not obliged to be automatically enrolled to more than double.