Epidemiological monitoring of illicit drug use has made significant progress in France since the late 1990s, as evidenced by the number of surveys conducted and their increased ability to accurately reflect current practices. For a significant proportion of individuals, adolescence and early adulthood is a period marked by experimentation with certain psychoactive substances; it also sometimes leads to the development of more regular use. The Health Barometer tracks changes in drug use between 1992 and 2010 among young people aged 15 to 30: analysis of these data thus complements the survey system established by the French Monitoring Center for Drugs and Drug Addiction (OFDT) and its partners regarding drug use among adolescents. Conducted via a self-administered and strictly anonymous questionnaire, the Survey on Health and Substance Use During the Military Service Preparation Call-Up (Escapad) provides a regular update on the levels of psychoactive substance use among 17- to 18-year-olds and highlights recent trends in usage patterns among late adolescents. The European School Project on Alcohol and Other Drugs (ESPAD) survey, for its part, tracks use among high school students, while the Health Behaviour in School-aged Children (HBSC) survey focuses on middle school students; both are conducted every four years. This set of surveys thus allows for the observation of the spread of substance use throughout adolescence into early adulthood, between the ages of 11 and 30. The substances studied in this chapter are either those classified on the list of controlled substances (cannabis, amphetamines, cocaine, ecstasy, LSD, heroin, poppers, hallucinogenic mushrooms), or substances diverted from their intended use for psychotropic purposes (glue, volatile solvents), with the characteristics of these various substances summarized in the box below. This chapter does not address “new synthetic substances” (NPS), which mimic the effects of various illicit substances (ecstasy, amphetamines, cocaine, cannabis, etc.) and appear to be on the rise (particularly synthetic cannabinoids). Alcohol and tobacco, due to their widespread use in France, are addressed in separate chapters (see the chapter “Alcohol consumption among 15- to 30-year-olds,” page 87, and the chapter “Smoking among 15- to 30-year-olds,” page 69) and will be addressed here only briefly, to be put into perspective with illicit drugs in a specific section dealing with poly-substance use. Since cannabis remains by far the most widely used illicit substance in France, a large portion of this chapter is devoted to it. The first part of this chapter presents the levels of cannabis use among young people aged 15 to 30, trends since the early 1990s, factors associated with use according to frequency of consumption, and the main regional differences. Major trends regarding other illicit drugs will then be outlined, concluding this overview with a brief description of regular poly-drug use practices.[chapter excerpt]