Attention-deficit/hyperactivity disorder (ADHD) is a neurodevelopmental disorder characterized by symptoms of inattention, impulsivity, and hyperactivity that interfere with social, occupational, or academic functioning. Clinical communities differ regarding whether or not formal psychological or neuropsychological assessment is necessary for diagnosing ADHD. A comprehensive assessment model that emphasizes differential clinical diagnosis is recommended. This model provides practice guidelines that ensure accurate diagnosis when faced with potential complicating factors, such as symptom overlap, substance abuse, and symptom feigning.