This paper provides a brief overview of the nature of attention-deficit/hyperactivity disorder (ADHD) in children and the current criteria used in its clinical diagnosis. While the disorder continues to be viewed as one of inattention and/or hyperactive-impulsive behavior, theories of ADHD are beginning to focus more on poor inhibition and deficient executive functioning (self-regulation) as being central to the disorder. Problems have been identified by research pertaining to the clinical diagnostic criteria outlined in the DSM-IV that, at present, remain unresolved. Clinicians should be aware of these problems and the adjustments that need to be made to them when dealing with special populations that were not represented in the field trials used to develop these criteria.