The study focuses on individuals who have consecutively presented to participating clinics with suspected attention and/or behavior concerns and would receive a clinician's comprehensive evaluation for ADHD. The study population includes male and female participants aged 6-17.99 years regardless of race, ethnicity, and socioeconomic origin. Because of inclusion of patients with consecutive presentation to provide a representative clinical sample, the exact numbers for sub-groups will not be controlled. The study population will include patients with comorbid conditions. The non-ADHD subgroup will include patients with other causes of attentional concerns, which may include other common psychiatric disorders, organic causes of attention problems (such as poor hearing, vision disturbances, remote head injury, substance abuse), and/or no diagnosis.