The high rate of co-occurring mental health issues and substance-use disorders has been well documented. The primary objective of this research was to evaluate the association between use of alcohol, marijuana, and other illicit drugs, and unmet mental health need and service use. A secondary aim of this research was to determine if the observed patterns of alcohol, marijuana, and other illicit drugs use and unmet mental health need and mental health service use are consistent with a theory of self-medication theory. On the latter view, people use psychoactive substances as a self-regulation strategy to alleviate distress. Research was conducted through secondary analysis of 2014 National Survey on Drug Use and Health (NSDUH) data. Overall, study findings supported the established correlation between mental health issues and substance use as well as a theory of self-medication. This study focused on people 18-25 years of age.