During the past two decades, California experienced a 25-fold increase in the number of drug offenders sentenced to State prisons. Harsher sentencing statutes have expanded the pool of prison-eligible offenders and promoted incarceration as a primary response to illicit drug use. California's uniquely harsher approach to drug crime is founded on deterrence and incapacitation theory, which promotes increased arrests, prosecutions, and prison sentences as the primary means to dissuade drug use and reduce street crime by removing the drug-involved offender from the community. The application of California's harsh sentencing measures, however, has not been uniform among California counties. This study tested deterrence and incapacitation theory by analyzing whether the counties that adopted strict enforcement approaches had the greatest declines in drug-related crime and drug abuse. The study found that over the last two decades, counties that sharply increased their imprisonment rates for drug offenses showed significantly slower decreases in the most serious Part I felony offenses, especially property offenses, than counties with more lenient approaches. Similarly, counties that energetically prosecuted and imprisoned more people for drug possession did not experience greater reductions in serious crime. Conversely, counties that adopted more balanced approaches, with less emphasis on arresting and imprisoning low-level drug users, showed significantly larger declines in property crime and larger (albeit not statistically significant) declines in violent crime as well. The absence of differential effects between counties with strict drug enforcement policies and counties with more lenient drug enforcement policies does not support the deterrence and incapacitation arguments of drug enforcement advocates. Extensive tables, 8 references, and appended county-by-county analysis