The association between cannabis and depression has received less attention than the links between cannabis use and psychosis. Some have suggested that cannabis use may be a contributory cause of suicidal behaviors. A number of studies have found association between cannabis use and suicide, but the quality of control for confounding variables has varied widely. There is increasing evidence that regular cannabis use and depression occur together more often than we might expect by chance. The association between cannabis use and depression may arise because the same factors that predispose people to use cannabis also increase their risk of depression. Cross-sectional and longitudinal studies have provided mixed evidence on the association between cannabis use and depression. There is a need for longitudinal and twin studies that assess the relationship between cannabis use, depression and confounding factors.