The impacts of Three Strikes on crime in California and throughout the U.S. are analyzed using cross-sectional time series analysis of state-level data from 1986 to 2005. The model measures both deterrence and incapacitation effects, controlling for pre-existing crime trends and economic, demographic, and policy factors. Despite limited usage outside California, the presence of a Three Strikes law appears to be associated with slightly but significantly faster rates of decline in robbery, burglary, larceny, and motor vehicle theft nationwide. Three Strikes is also associated with slower declines in murder rates. Although California’s law is the broadest and most-frequently-used Three Strikes policy, it has not produced greater incapacitation effects on crime than other states’ far more limited laws. The analyses indicate that the “toughest” sentencing policy is not necessarily the most effective option.