Several studies have shown that Δ-9-THC the main psychoactive constituent of cannabis, can impair cognitive functions, especially attention, episodic memory, working memory and executive functions. These impairments have been related to the duration, frequency, dose and age at onset of cannabis use. Cognitive deficits may disappear with abstinence, but abnormalities may be long-lasting in subjects who began smoking cannabis before age 15. The lifetime prevalence of cannabis use disorders is about 1% in the general population. The main characteristics of cannabis use disorders are craving, persistent desire or unsuccessful efforts to cut down or control cannabis use, and persistent avoidance of familial, social occupational or recreational activities because of cannabis use. Nine prospective longitudinal studies in the generalpopulation have shown that cannabis use is associated with a two-fold increase in the risk of psychotic disorders, particularly schizophrenia, compared to controls. The risk of psychosis increases in a dose-related fashion. A higher risk of schizophrenia is predicted by earlier onset of cannabis use. The effects of cannabis are exerted primarily through THC interaction with cannabinoid (CB) 1 receptors in the brain. Cannabis exposure may disrupt the last steps of brain maturation, through the endocannabinoid system, thereby increasing the risk of psychosis during adolescence.