ADHD is the most common psychiatric disorder of childhood (3% to 5% of children) with continued morbidity into adolescence (85%) and adulthood (50% to 70%). It is a condition associated with widespread significant impairment in academic, occupational, social, and emotional functioning. It is also a condition with a broad extensive differential diagnosis as well as a high rate of comorbidity. There is no diagnostically definitive test for ADHD. Therefore, assessments for ADHD need to be comprehensive and should involve multiple domains, informants, methods, and settings. The comprehensive assessment needs to determine whether the subject has ADHD or another disorder. Thus, evaluation of various organic conditions, functional disorders, developmental status, situational, environmental, and family problems should all be explored. The clinical interview of the child and family is one of the cornerstones of the assessment process. A comprehensive medical history and examination, psychoeducational tests, and school-related evaluation, as well as a view of the child's social and emotional functioning, are also crucial. A wide array of rating scales, tests, and measures have been developed (see Table 1) to aid in the systematic standardized assessment of the various deficits associated with ADHD. None of these tests is definitive, however. Recent development of tests of executive functioning, neuroimaging, and genetics may provide more exact diagnostic guidelines in the future. Current DSM-IV criteria are phenomenologic rather than etiologic and are much more relevant and appropriate for children with ADHD compared with adolescents and adults. A comprehensive assessment involving various domains of functioning (academic, social, emotional, physical, and familial) provide not only more accurate diagnosis but also directions as to what difficulties exist and what multifaceted treatment plan is needed to produce current improvement and long-term positive outcome.