SUMMARY: When policy makers think about how to get people to adhere to the law, they often think about increasing the certainty and severity of punishment. Most criminological research on deterrence has shown that certainty of apprehension, rather than severity of punishment is more likely to produce deterrence. However, the likelihood of being apprehended for a minor cannabis offence is so low that it is unlikely that variables such as certainty, celerity, or severity will have much impact on use. This chapter summarises the research literature on deterrence and employs data from Australian research on the social impacts of a conviction for a minor cannabis offence to explain why criminal penalties are not a major deterrent to cannabis use. It shows, however, that the social impacts of receiving a criminal conviction for such an offence can be considerable. A range of other variables such as public attitudes to use, the perceived fairness of the law and its enforcement, peer influences, and the utility of cannabis use are likely to far outweigh the deterrent value of a criminal conviction. It concludes that the application of the criminal law to prevent cannabis use is an inefficient and ineffective use of valuable and scarce criminal justice resources. Rather, prohibition with civil penalties should maintain any general deterrent effect while reducing individual and community costs of conviction.