What is attention deficit hyperactivity disorder (ADHD)?

Attention-Deficit/Hyperactivity Disorder (ADHD) is a behavior disorder, usually first diagnosed in childhood, which is characterized by persistent problems with inattention and/or impulsivity and hyperactivity.  These difficulties must be present in two or more settings (e.g., home, school, work, social), and must occur with sufficient frequency and severity as to interfere with the person's ability to function in an age-appropriate manner.

What are the different types of ADHD?