Between 1991 and 2001, crime rates dropped by about a third across all crime categories. We suggest that the introduction and growth of mobile phone technology may have contributed to the crime decline in the 1990s, specifically in the areas of rape and assault. Given that mobile phones increase surveillance and the risks of apprehension when committing crimes against strangers, an expansion of this technology would increase the costs of crime as perceived by forward-looking criminals. We use the available mobile phone data to show that there is a strongly negative association between mobile phones and violent crimes, although data limitations preclude us from being able to make any claims about causality. We show how the intuition about mobile phones providing crime deterrence fits in well with modern discussions in the crime literature regarding optimal policy and the expanding use of private security precautions in crime prevention.