Marijuana (cannabis) is the most commonly used illicit substance in America. However, more and more findings are revealing altered brain structure and function associated with such substance use. The critical question surrounding cannabis use in neuroscientific research is if these brain alterations are a cause or a consequence of cannabis use. Solving such questions could lead to more informed decisions regarding the safety and legality of the substance’s use and the development of appropriate interventions and rehabilitation for users. Determining causality in research can take years, even decades, but in the meantime, the current research is providing valuable insight into the possible effects of the substance.