Attention-Deficit/Hyperactivity Disorder (ADHD) is the most commonly diagnosed neurobehavioral disorder in childhood. Children with ADHD struggle in school due to problems with attention and high levels of impulsivity and hyperactivity. They are at substantially increased risk for long-term difficulties into adulthood, including academic underachievement, substance abuse, and criminal behavior. The diagnosis of ADHD, which is based on subjective ratings by parents and teachers, likely results from multiple different, overlapping differences in circuits of the brain responsible for attention and impulse control. However, we do not have any scientific or clinical tests that allow us to understand these circuits. In an effort to improve ADHD outcomes, we have used a technology called Transcranial Magnetic Stimulation (TMS) to identify highly reliable measurements of brain function. We have identified two very promising measures that are abnormal in children with ADHD and, importantly, also predict the severity of ADHD behaviors. The goal of this project is to determine if these two TMS measurements could be used to help better guide ADHD treatment. To do this, we will perform three investigations in 8 to 12 year old children to determine: 1) test-retest reliability; 2) pharmacologic responsiveness; and 3) correlations with two domains of function relevant to ADHD: Cognitive Control and Emotional Valence. Through these investigations, we aim to determine whether these two TMS brain measures are reliable and meaningful enough to be used to help improve precision of individually-targeted and effective ADHD treatments.