This document summarizes attention deficit hyperactivity disorder (ADHD), including its three subtypes, potential causes, and common symptoms. It notes that ADHD is characterized by problems with concentration, impulse control, organization and memory. The three subtypes are combined, inattentive, and hyperactive-impulsive ADHD. Genes, environmental factors, and brain injuries may contribute to ADHD. Symptoms of inattention, hyperactivity and impulsivity often become apparent when children enter school, where they may have trouble paying attention, completing tasks, and sitting still. Boys are more at risk than girls, with average age of onset at 7 years old.