Public policy and scientific knowledge concerning deterrence have long been marching in different directions. Despite the proliferation of three-strikes, mandatory minimum, and concealed weapons laws and retention of capital punishment in 37 states, there is little credible evidence that changes in sanctions affect crime rates, and there is no credible evidence that capital punishment deters better than life sentences or that allowing citizens to carry concealed weapons deters at all. There is evidence that changes in enforcement and sanctions can affect some kinds of behavior - for example, tax compliance, speeding, illegal parking - and there are plausible grounds for believing that other deterrable behaviors can be identified. Doing so will require fine-grained studies that take account of offender characteristics and perceptions, offending situations, and whether and how new enforcement strategies and sanctions systems are implemented.