Attention deficit hyperactivity disorder (ADHD) is a group of behavioral, emotional and cognitive abnormalities in childhood. It has been known as mild brain dysfunction, attention deficit hyperactivity disorder, and ADHD. It is found in about 5% of school-age children and is more common in boys than in girls. The cause is not well understood. Current research suggests that the disease is a syndrome caused by a variety of biological, psychological and social factors. The pathogenesis also lacks a satisfactory explanation and consensus. Recent studies suggest that it may be related to the underfunctioning of the dopamine system in the brain and genetic defects. In this paper, we only provide an overview of its clinical manifestations and behavioral characteristics.