Objective: This article aims to identify the sociodemographic and psychosocial factors (based on the Theory of Planned Behavior) among young people that are associated with intentions to remain tobacco-free and not to experiment with cannabis, as well as with intentions to reduce alcohol consumption during a single evening, to quit smoking, and, finally, to reduce cannabis use among current users. Methods: 3,652 young people aged 15 to 25 were surveyed via an online questionnaire. The sample was recruited from an online access panel and constructed using the quota method applied to the variables of gender, age, region, housing type, and socioeconomic status of the head of household. Results: Each of the psychosocial determinants (attitudes, norms, and control) contributes to explaining the intention to remain a non-user of tobacco and/or cannabis. The presence of a prohibition norm within the family is also associated with a lower intention to experiment with cannabis. Among users, attitude determines the intention to quit or reduce consumption of the three substances (tobacco, alcohol, and cannabis); control influences the intention to reduce alcohol consumption and quit tobacco use. Finally, the norm influences the intention to reduce alcohol consumption during a single evening. Conclusion: Our analyses demonstrate the value of measuring psychosocial factors to explain health behaviors, as well as to identify and propose courses of action. They reinforce the priority placed on tobacco prevention, justify the focus on early intervention, and reaffirm the importance of considering social health inequalities in the development of prevention programs.[author’s abstract]