Children with ADHD and age and sex-matched healthy controls will be scanned at the beginning of the study. In this first part of the study, MRI findings will be compared between the two groups to see whether brain functioning of children with ADHD differs from that of healthy children. In the second part of the study, the children with ADHD will meet weekly with the study doctor and be administered either extended-release guanfacine (Intuniv) or lisdexamfetamine (Vyvanse) for 6 weeks. Selection into either treatment group will be randomly determined. At the end of the study, the children with ADHD treated with either medication will have a second MRI scan. The findings of that second MRI scan will be compared to the first MRI scan in order to examine brain changes due to the medication. These second MRI scans will also be compared to the scans of the healthy controls.