In conclusion, complex medical and psychiatric comorbidity is com-mon in individuals with substance use disorders. It is important to assess comorbidity because of the implications for prevention and treatment. Studies of the neurobiology of substance use and psychiatric disorders are accumulating rapidly and informing treatment development. Information about the prevention and treatment of infectious diseases and other medical conditions associated with substance use disorders also is growing, and it is important that patients are able to benefit from this. The articles in this issue provide state-of-the-art information about several issues related to comorbidity in substance use disorders.