In general, recommendations for the DSM-V and future diagnoses of psychiatric disorders include a dimensional approach to complement the standard categorical approach. For the assessment of attention-deficit/hyperactivity disorder (ADHD), dimensional approaches to supplement the rigid categorical approach of the DSM-IV abound. Historically, dimensions based on severity of symptoms of ADHD and severity of general psychopathology have been used. General dimensional approaches described by a workgroup organized by the American Psychiatric Association are reviewed to provide background and context for a discussion of old and new dimensional approaches to complement future categorical diagnosis of ADHD in the DSM-V.