Diagnosis and management of attention-deficit/hyperactivity disorder (ADHD) in adults is complex and challenging because of the frequent comorbidity of other psychiatric disorders that have symptoms overlapping with those of ADHD. The presence of comorbidities can create challenges to making an accurate diagnosis and also impact treatment options and outcomes. This review discusses disorders that may be comorbid with ADHD in adults, including anxiety, mood, substance use disorder, antisocial personality disorder, and borderline personality disorder. Suggestions for recognizing these comorbidities and distinguishing them from ADHD and perspectives on their possible impact on ADHD treatment are included. Adjunctive nonpharmacologic modalities may be especially helpful in the case of comorbid mood, anxiety, substance abuse, or personality disorders.