Attention deficit hyperactivity disorder (ADHD) is a common mental health disorder that is characterized by excessive levels of hyperactivity and impulsive behavior. Many ADHD symptoms such as high activity levels, difficulty remaining still for long periods of time, and limited attention span, are common to young children in general. The primary difference in children with ADHD is that their restlessness and inattention are noticeably greater than is expected for their age, and that their behavior interferes with functioning at home, with friends, or at school. ADHD is diagnosed as one of three types, inattentive type, hyperactive type, or a combination of both. Those with a combination type exhibit several characteristics of both types.