Additional Study Information: Children with Attention-Deficit/Hyperactivity Disorder (ADHD) are typically treated with two types of medications: stimulants and non-stimulants. These medications work in different ways to treat ADHD symptoms, but we do not fully understand the brain mechanisms underlying the medications' effects. This study will use MRI scanning to look at changes in the brain associated with medication treatment of ADHD in children. In this study, we will be comparing two FDA-approved medication treatments for ADHD in children: (1) A stimulant called Vyvanse (lisdexamfetamine); (2) A non-stimulant medication called Intuniv (extended-release guanfacine). After an initial evaluation and MRI scan, children with ADHD will be randomized to receive 6 weeks of treatment with Vyvanse or Intuniv. Children and their parents will meet weekly with the doctor to adjust and maintain the medication dose. After 6 weeks, children will receive a second evaluation and MRI scan. Healthy control children will also be recruited, but they will only receive an evaluation and MRI over the course of one or two visits to our offices. Families will be compensated for their time.