Psychoses are a group of diseases that cause enormous health and emotional burden for the patient and their families. In recent decades, the need for early intervention to improve their prognosis has been highlighted. It has long been postulated that cannabis use is a risk factor added to other factors such as genetics and developmental abnormalities. For this reason, and in the light of a growing movement for legalization of its use, it is vitally important to have reliable information to understand the details of the relationship between cannabis abuse and the development of psychosis. The purpose of this paper is to review the available information on the subject. Beyond some methodological criticisms, epidemiological fndings provide strong evidence that cannabis use may increase the risk of psychotic disorders, a public health message that should be spread.