Many adults with a diagnosed psychiatric disorder also have attention-deficit/hyperactivity disorder (ADHD). In many cases, comorbid ADHD is unrecognized and/or undertreated. Differential diagnosis of adult ADHD can be challenging because ADHD symptoms may overlap with other psychiatric disorders and patients may lack insight into their ADHD-related symptoms. Current ADHD diagnostic criteria in the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition-Text Revision may prevent appropriate diagnosis of many patients with significant ADHD symptoms. Adults may not be able to provide a history of onset of symptoms during childhood, and it may be difficult to confirm that ADHD symptoms are not better accounted for by other comorbid psychiatric conditions. Comorbid ADHD is most prevalent among patients with mood, anxiety, substance use, and impulse-control disorders. ADHD can negatively affect outcomes of other comorbid psychiatric disorders, and ADHD symptoms may compromise compliance with treatment regimens. Furthermore, unrecognized ADHD symptoms may be mistaken for poor treatment response in these comorbid disorders. In these individuals, ADHD pharmacotherapy seems to be as effective in reducing core ADHD symptoms, as it is in patients who have no comorbidity. Limited evidence further suggests that ADHD therapy may help to improve symptoms of certain psychiatric comorbidities, such as depression. Therefore, management of ADHD may help to stabilize daily functioning and facilitate a fuller recovery.