The article examines the causal association between cannabis use and psychosis, using established criteria of causality. Five studies were identified that met the criteria of having well-defined samples from population-based registers or cohorts and using prospective measures of cannabis use and adult psychosis. The results show that cannabis use confers a twofold increase in the relative risk for later schizophrenia at the individual level, and eliminating cannabis use could reduce the incidence of schizophrenia by approximately 8% at the population level. However, cannabis use is neither a sufficient nor a necessary cause for psychosis but is part of a complex constellation of factors leading to psychosis. The article concludes that discouraging cannabis use among vulnerable youths could prevent cases of psychotic disorder, and further research is needed to understand the mechanisms by which cannabis causes psychosis.The article examines the causal association between cannabis use and psychosis, using established criteria of causality. Five studies were identified that met the criteria of having well-defined samples from population-based registers or cohorts and using prospective measures of cannabis use and adult psychosis. The results show that cannabis use confers a twofold increase in the relative risk for later schizophrenia at the individual level, and eliminating cannabis use could reduce the incidence of schizophrenia by approximately 8% at the population level. However, cannabis use is neither a sufficient nor a necessary cause for psychosis but is part of a complex constellation of factors leading to psychosis. The article concludes that discouraging cannabis use among vulnerable youths could prevent cases of psychotic disorder, and further research is needed to understand the mechanisms by which cannabis causes psychosis.