Youth's perceptions of the morality of alcohol and other drug use and the perceived legitimacy of laws regulating such use have received scant attention in the international public health literature. To date, the focus has mainly been on emphasizing the health and social disbenefits of substance use in an attempt to counter the perceived psychological benefits (positive expectancies) of use and peer reinforcement. Following exploratory qualitative research, a structured questionnaire was administered to a sample of 611 youths aged 14-17 years. Analysis of the data found that use of alcohol, tobacco and marijuana was directly related to moral perceptions: those considering use as "wrong under any circumstance" were less likely to be users than those who considered it "ok under some or any circumstance". Substance use was also related to legitimacy perceptions: those who thought laws relating to alcohol, tobacco and marijuana use were justified were less likely to be users than those who thought these laws were not justified. The implications of these findings for future research and for the design of more effective intervention strategies are discussed. It is suggested that interventions including student discussion of the moral and legal issues surrounding substance use may prove effective in postponing or even preventing substance use, particularly tobacco and marijuana consumption, or reducing the excess use of these substances.