There is an ongoing American policy debate about the appropriate legal status for psychoactive drugs. Prohibition, decriminalization, and legalization positions are all premised on assumptions about the behavioral effects of drug laws. What is actually known and not known about these effects is reviewed. Rational-choice models of legal compliance suggest that criminalization reduces use through restricted drug availability, increased drug prices, and the deterrent effect of the risk of punishment. Research on these effects illustrates the need for a more realistic perspective that acknowledges the limitations of human rationality and the importance of moral reasoning and informal social control factors. There are at least 7 different mechanisms by which the law influences drug use, some of which are unintended and counterproductive. This framework is used to explore the potential behavioral effects of decriminalization and legalization.