Attention-Deficit/Hyperactivity Disorder (ADHD) has become the most commonly diagnosed form of psychopathology in the preschool years. To address this alarming trend, the overall goal of the proposed investigation, Development of ADHD in Preschool Children: Neuroimaging and Behavioral Correlates is to characterize the anomalous early development (brain, cognition, and behavior) of preschool children identified as at risk for ADHD, in order to better understand the neurodevelopmental pathways that lead to behavioral dysfunction and ultimately the diagnosis of ADHD, as well as those associated with normalization of behavior. This study proposes to uncover a critical link between brain and behavioral development in ADHD, which will contribute to identification of important early biomarkers for the disorder. Preschool children who present with symptoms of ADHD are at significant risk for social and academic dysfunction. Of particular concern are observations that preschoolers with ADHD have similar patterns of functional impairment as school-age children with ADHD, and that few are well-adjusted by adolescence, including higher rates of emergency room visits, and increased health care costs and societal costs. The costly toll that ADHD takes on individual adjustment, family life, schools, health care, and social services underscore the importance of understanding its developmental course, with the ultimate goal of earlier identification and treatment. Despite being a disorder (presently) requiring symptom onset by age 7 years, there have been few studies of longitudinal brain development in preschoolers presenting with symptoms of ADHD, and none involving neuroimaging. Consequently, the proposed study will use a longitudinal design in a mixed model of imaging and behavioral variables, to delineate and contrast the growth (from ages 4 to 7 years) of children identified as at risk for ADHD with that of age-matched typically developing children without symptoms of ADHD. Aim #1: To determine by quantitative MRI (aMRI), and as expressed in neuropsychological assessment, the anomalous development of brain regions (subcortical and cortical) and behavior among preschool children identified as at risk for ADHD. Aim #2: As a first step in identifying those preschoolers most at risk, we will determine the trajectory of development (from ages 4 to 7 years) of brain regions and associated neuropsychological functioning among preschool children identified as at risk for ADHD.