Attention deficit hyperactivity disorder is one of the most common behavioral disorders of childhood and is frequently assessed and treated by pediatricians and other primary care physicians. The diagnosis of attention deficit hyperactivity disorder is based on the careful synthesis of clinical data derived from multiple sources, notably, the patient, parents, and teachers. Standard behavioral rating scales simplify and standardize collection of clinical data but by themselves are not sufficient for a diagnosis. Recognition and treatment of disorders comorbid with attention deficit hyperactivity disorder are necessary for optimal outcomes. Psychoeducational testing is useful in the assessment of suspected associated learning disabilities, but there is no evidence at present to support use of psychologic testing, laboratory measures of attention, electroencephalography, or neuroimaging studies in the clinical assessment of attention deficit hyperactivity disorder.