Anxiety disorders are the most common form of pediatric psychiatric illness, affecting up to 30% and severely impairing up to 20% of all youth prior to age 18. Unfortunately, up to 50% of children remain symptomatic even with best available treatments, making anxiety disorders a major public health problem. To devise new treatments for anxiety disorders, more research is needed into underlying brain mechanisms. Research studying mechanisms suggests that anxiety disorders are linked to alterations in attention, including increased attention to threatening stimuli (e.g., feared objects or sounds). However, more recent research suggests these attention alterations may be due to a larger problem in which attention is increased to all stimuli (e.g., any loud noise, flash of light, etc.). This alteration in attention seems to be the opposite of ADHD, the prototypical disorder of attention in childhood and also a common childhood psychiatric disorder. In ADHD, children appear to have a generalized decrease in attention to suddenly appearing stimuli (e.g., children with ADHD might not hear their name called). Intriguingly, although anxiety and ADHD appear to demonstrate opposing attention- related problems, these disorders occur in the same child more often than expected by chance. Understanding the brain mechanisms underlying attention alterations in pediatric anxiety and ADHD is critical in designing new treatments that attempt to target or ‘correct’ problems in these brain mechanisms. In this study, we test the hypothesis that attention-related brain circuitry is overactive in anxiety disorders such that bright objects and errors elicit increased activity compared to children with no disorder. We further predict this circuitry is underactive in ADHD such that bright objects and errors elicit diminished activity increases. We hypothesize that children with both anxiety and ADHD have an intermediate level of activity (similar to children with no disorder), though less is known in this area. Finally, we predict that peer observation (a mild threat/stressor) further exacerbates the overly active attention-related brain circuitry in children with anxiety. To test these hypotheses, we use cutting-edge neuroscience tools to provide a nuanced characterization of attention-related brain circuitry in N=300 children ages 10-12 years with anxiety disorders (n=75), ADHD (n=75), both anxiety and ADHD (n=75), and no psychiatric disorder (n=75). Children play a computer game during which we measure how suddenly appearing objects capture their attention. Brain activity is measured using functional MRI, pinpointing the specific brain locations that are disrupted; and electroencephalography (EEG), providing precise timing information. We characterize attention-related brain circuitry when children are being observed by a peer versus without this stressor. This study will provide a comprehensive description of circuit-level mechanisms of altered attention in pediatric an...