Recent research on the prevalence of defensive gun use has prompted growing concern that government efforts to regulate gun ownership and use may be counterproductive. Estimates of defensive gun use from the National Crime Victimization Survey (on the order of 100,000 per year) appear to be too low. However, estimates from one-time telephone surveys (from 1.5 to 2.5 million per year) appear to be too high; even a modest rate of false positives may lead to substantial upward bias. A more promising approach is to examine the net effects of gun policies on rates of crime and injury directly. Evidence for a substantial deterrent effect of permissive concealed gun-carrying laws comes from a recent study by Lott and Mustard. Reanalysis of their data suggests that the estimated “treatment effects” are due in part or whole to unmeasured variables. More recent studies find no evidence of a significant negative effect of these laws on crime, though the available research remains far from definitive.