In recent years, there has been an increased recognition of the common comorbidity of attention-deficit/ hyperactivity disorder (ADHD) and substance use disorder (SUD) among adolescents and adults. ADHD can be an important factor in the pathogenesis and maintenance of SUD; moreover, retrospective studies suggest that treating ADHD during childhood may prevent the development of SUD. In addition, treatment of ADHD among adults, and possibly adolescents, with SUD can reduce their risk of relapse. Theoretical mechanisms that may explain the relationship between ADHD and SUD are explored in this paper. Current research and recommended clinical practices related to the diagnosis and treatment of ADHD with SUD in adolescents are discussed as well. More research is needed to definitively assess the effectiveness and safety of medications in this population of youths with ADHD and SUD.