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Since 1988, every US state has established a database of criminal offenders' DNA profiles. These databases have received widespread attention in the media and popular culture, but this paper provides the first rigorous analysis of their impact on crime. DNA databases are distinctive for two reasons: (1) They exhibit enormous returns to scale, and (2) they work mainly by increasing the probability that a criminal is punished rather than the severity of the punishment. I exploit the details and timing of state DNA database expansions in two ways, first to address the effects of DNA profiling on individual's subsequent criminal behavior and then to address the impacts on crime rates and arrest probabilities. I first show that profiled violent offenders are more likely to return to prison than similar, unprofiled offenders. This suggests that the higher probability of getting caught outweighs the deterrent effect of DNA profiling. I then show that larger DNA databases reduce crime rates, especially in categories where forensic evidence is likely to be collected at the scene - e.g., murder, rape, assault, and vehicle theft. The probability of arresting a suspect in new crimes falls as databases grow, likely due to selection effects. Back-of-the-envelope estimates of the marginal cost of preventing each crime suggest that DNA databases are much more cost-effective than other common law enforcement tools.