The link between cannabis use and psychosis has been studied intensively and debated hotly for decades. The authors review the research that has been done on the topic, including the studies which produced evidence of a relationship between the two phenomena, and give a detailed analysis of the hypotheses about the nature of the link (the direction of causality). According to the reviewed literature an increased prevalence of cannabis use can be found in the schizophrenic population. The use of the drug is associated with a worse prognosis and an earlier onset of schizophrenia. However, findings about the possible effect of cannabis use on the specific symptoms of schizophrenia have been contradictory. An association has also been observed between cannabis use and schizotypal personality traits, but evidence for a specific, cannabis induced functional psychosis is still lacking. Based on the data of longitudinal studies, cannabis use should be recognized as a risk factor for later psychosis. The abuse of the drug increases the likelihood of later psychotic symptoms especially among individuals with vulnerability or when the use starts in early adolescence. At the same time, the role of self-medication or a common genetic background cannot be excluded either, and a circular causality is very possible.