Attention-deficit/hyperactivity disorder is the most common neurobehavioral disorder of childhood, with most children maintaining symptoms of ADHD as adolescents and as adults. It is among the most common chronic conditions that primary care pediatricians see. Progress in brain imaging, genetics, neuropsychology, and molecular biology of ADHD reveals complex interactions between neurologic mechanisms, genetics, and environmental influences. Core behaviors seen in children with ADHD include hyperactivity, impulsivity, and inattention. The DSM-IV standard for diagnosis emphasized the importance of documenting that these behaviors occur at school and in the home, the chronicity of symptoms, and the connection between the behaviors and impairments in educational achievement or social development. The complex nature of this disorder is indicated by the observation that children and adolescents with ADHD have associated conditions, such as learning disorders, anxiety, oppositional behaviors, and depression. The AAP practice guideline on the evaluation and diagnosis of school-aged children with ADHD provides primary care pediatricians with a scientific foundation for assessing children with behavior and learning problems. Attention to accurate documentation of behaviors, coexisting learning disorders and mental health conditions, and the development of office practices that support the diagnostic process will ensure that children receive an appropriate and comprehensive assessment.