Commentators covering recent social movements, such as the Arab Spring, have claimed that cell phones and social media enable collective action. We develop a theoretical model to illustrate why, focusing on two mechanisms: first, by enabling communication among would-be protesters, cell phones lower the costs of coordination; second, these technologies broadcast information about whether a protest is repressed. Knowing that a large audience will now witness, and may be enraged by repression, governments refrain from squashing demonstrators, lowering the cost of protesting. We evaluate the model’s predictions using high-resolution global data on the expansion of cell phone coverage and the incidence of protest from 2007-2014. Our difference-in-differences estimates indicate that cell phone coverage increases the probability of protest by two times the mean. Consistent with our second mechanism, we also find that the expansion of coverage reduces the probability of repression.