What is Attention Deficit Hyperactivity Disorder?1 Attention Deficit Hyperactivity Disorder, ADHD, is one of the most common mental disorders that develop in children. Children with ADHD have impaired functioning in multiple settings, including home, school, and in relationships with peers. If untreated, the disorder can have long-term adverse effects into adolescence and adulthood. More about ADHD Signs & Symptoms Symptoms of ADHD will appear over the course of many months, and include: *