Cannabis use is extremely prevalent in young people and there is controversy over the psychosocial harm of this use. One risk stands out--that of the possibility of cannabis being a risk factor of an illness as serious and incapacitating as schizophrenia. An avalanche of information has emerged recently on this subject, which it is essential to review. There are a number of cohort studies that support the idea of cannabis use as inducing psychotic symptoms and a precipitating factor of schizophrenia. These studies haves some methodological limitations, which we subject to critical analysis, such as the different outcome evaluations, heterogeneity in the measurements of vulnerability to the psychosis, the difficulty in controlling the possible confusion factors and the inadequate interpretation of the results obtained. In conclusion, we consider that the use of cannabis is clearly associated with the inducement of psychotic symptoms and is, possibly, a risk factor of schizophrenia in people with a genetic or psychosocial vulnerability, preventive methods being necessary in high-risk groups. These are fundamentally those of users of large quantities of cannabis and those who initiate use in adolescence.