Children with Attention-Deficit/Hyperactivity Disorder (ADHD) are typically treated with two types of medications with differing mechanisms of action: stimulants and non-stimulants. The stimulant Vyvanse (lisdexamfetamine, LDX), and the non-stimulant Intuniv (extended-release guanfacine, GXR), are both FDA approved treatment for ADHD. Clinical trials have shown that both medications are effective in reducing ADHD symptoms, although the neurobiological mechanisms by which Vyvanse and Intuniv produce these effects remain unknown. The aim of this study is to examine the mechanisms by which LDX and GXR reduce symptoms in patients with ADHD. MRI scanning will be used to identify treatment-related changes in brain structure and function.