Comorbidity is a topic that our stakeholders–patients, family members, health care professionals, and others– frequently ask about. It is also a topic about which we have insufficient information, and so it remains a research priority for NIDA. This Research Report provides information on the state of the science in this area. And although a variety of diseases commonly co-occur with drug abuse and addiction (e.g. HIV, hepatitis C, cancer, cardiovascular disease), this report focuses only on the comorbidity of drug use disorders and other mental illnesses.