Attention-deficit/hyperactivity disorder in childhood can persist into adulthood in at least 30 percent of patients, with 3 to 4 percent of adults meeting the Diagnostic and Statistical Manual of Mental Disorders, 4th ed., diagnostic criteria. A number of conditions, such as thyroid disease, mood disorders, and substance use disorders, have symptoms similar to those of attention-deficit/hyperactivity disorder and should be considered in the differential diagnosis. Steroids, antihistamines, anticonvulsants, caffeine, and nicotine also can have adverse effects that mimic attention-deficit/hyperactivity disorder symptoms. Proper diagnosis and treatment can improve daily functioning. Diagnosis relies on a thorough clinical history, supported by a number of rating scales that take five to 20 minutes to complete, depending on the scale. Clinical guidelines recommend stimulants and the nonstimulant atomoxetine as first-line treatments, followed by antidepressants. Cognitive behavior therapy has also been shown to be helpful as adjunctive treatment with medication. For adults with coexisting depression, the combination of an antidepressant and stimulants has been shown to be safe and effective. To monitor for misuse or diversion of stimulants, family physicians should consider using a controlled substances agreement and random urine drug screening in addition to regular follow-up visits.