Cannabis is the most popular illegal drug. Its legal status is typically justified on the grounds that cannabis use has harmful consequences. Empirically investigating this issue has been a fertile topic for research in recent times. We provide an overview of this literature, focusing on studies which seek to establish the causal effect of cannabis use on health, education, and labor market success. We conclude that there do not appear to be serious harmful health effects of moderate cannabis use. Nevertheless, there is evidence of reduced mental well-being for heavy users who are susceptible to mental health problems. While there is robust evidence that early cannabis use reduces educational attainment, there remains substantial uncertainty as to whether using cannabis has adverse labor market effects.