Adolescents with attention-deficit/hyperactivity disorder (ADHD) are nearly three times more likely to develop substance use disorders (SUD) than their typically developing peers, yet there currently are no biomarkers that can help clinicians identify which ADHD patients are at higher SUD risk and thus, who may benefit most from SUD prevention. This is a critical problem, given that ADHD psychostimulant medications ? which are Schedule II Controlled Substances ? are misused and abused by 26.7% of this population. In this research study, non-invasive MRI methods will be used to examine brain structural connectivity (i.e., number of neural fiber connections) and brain iron (as an index of dopamine) as possible brain biomarkers of increased SUD risk in ADHD.