Using a randomized experiment in a public Swiss university, we study the impact of online live streaming of lectures on student achievement and attendance. We find that (i) attending lectures via live streaming lowers achievement for low-ability students and increases achievement for high-ability ones; (ii) students use the live streaming technology only occasionally, apparently when random events make attending in class too costly, and (iii) offering live streaming reduces in-class attendance only mildly. These findings have important implications for the effective design of education policies.