Commentators covering recent social movements, such as the Arab Spring, commonly claim that cell phones enable protests. Yet, existing empirical work does not conclusively support this contention: some studies find that these technologies actually reduce collective action; many others struggle to overcome the selection problems that dog observational research. We propose two mechanisms through which cell phones affect protests: (1) by enabling communication among would-be protesters, cell phones lower coordination costs; and (2) these technologies broadcast information about whether a protest is repressed. Knowing that a larger audience now witnesses and may be angered by repression, governments refrain from squashing demonstrations, further lowering the cost of protesting. We evaluate these mechanisms using high-resolution global data on the expansion of cell phone coverage and incidence of protest from 2007 to 2014. Our difference-in-differences estimates indicate that cell phone coverage increases the probability of protest by over half the mean. Consistent with our second mechanism, we also find that gaining coverage has a larger effect when it connects a locality to a large proportion of other citizens.