PROJECT SUMMARY Attention-deficit/hyperactivity disorder (ADHD) is the most commonly diagnosed neurodevelopmental disorder of childhood and affects approximately 5-10% of children worldwide. ADHD is characterized by core symptoms of inattention, hyperactivity, and impulsivity, though there is much heterogeneity in the ADHD phenotype. Much research into the specific neural dysfunction underlying the pediatric ADHD phenotype has employed categorical approaches, comparing children with ADHD to typically developing children. However, children with and without clinical ADHD diagnoses experience a wide range of ADHD-related symptoms. As individuals with ADHD symptom burden, regardless of diagnosis, are at risk for negative-long term outcomes, the proposed studies will investigate the ADHD phenotype categorically and dimensionally to increase the understanding of the neural substrates of ADHD symptom burden in clinical and nonclinical populations. As such, the proposed research will use a novel dynamic functional connectivity method to determine whether task-based variations in functional connectivity and behavior differ in children with ADHD and in children without ADHD (categorical approach; Specific Aim 1), and whether there are relationships among ADHD symptom burden, functional connectivity dynamics, and behavior in a large sample of children with continuous ADHD-related symptom measures (dimensional approach; Specific Aim 2). Exploratory Aim 3 will further probe the heterogeneity in ADHD symptom burden that exists within and across ADHD by using a community detection algorithm to identify subgroupings of individuals with and without ADHD that exhibit similar symptom profiles. Together, the three aims of the proposed research will contribute to the development of strategies that allow for the identification of individuals who may benefit from treatment and intervention for ADHD symptom burden across diagnostic categories.