Monitoring drug use among adolescents is a priority for public authorities, requiring the implementation of methods tailored to this sensitive issue. After situating epidemiological surveillance tools within the broader range of social science tools available, this study synthesizes the results of representative surveys of the adolescent population and the main trends observed in recent years. A survey of 17-year-olds and another conducted among the general population—the analysis of which here focuses on 18- to 25-year-olds—were considered. A school-based survey of 12- to 18-year-olds also allows for comparison with other European nations. The prevalence of psychoactive substance use during adolescence is rising rapidly, though with significant variations depending on the substance. Since 2000, smoking has declined sharply, while alcohol use has remained relatively stable, and instances of drunkenness appear to have increased between 2003 and 2005. The rise in cannabis experimentation and regular use, which was very strong during the 1990s, ceased during the 2000s. Finally, while ecstasy and cocaine have become more widespread among the population, levels of experimentation with them remain low, and those for other substances—which appear stable—are even less frequent.