In order to help children with ADHD achieve the best possible outcomes, it is important for doctors to study why this happens. One possible reason is development of tolerance to the medication. Tolerance means that a drug's effects decrease when it is taken consistently over time, so that an increased dose is needed to continue showing effects. Some doctors believe that children who take stimulant medication for ADHD develop tolerance to it which would explain why benefits may not persist over time, but no research studies have been done to measure whether this occurs. This study aims to see if children show a tolerance effect to stimulant medication and whether that tolerance can be prevented by taking short breaks from the medication called medication holidays.