When extreme, trait anxiety responses are indicative of later psychopathology development. Trait anxiety emerges in early childhood, likely dependent on postnatal maturation of neural systems that facilitate adaptive threat responses. In this Dissertation, we examine how dynamic functional organization of prefrontal-amygdala circuits during early-life contributes to development of threat-related anxiety responses. Using a nonhuman primate model, we longitudinally characterized 35 rhesus monkeys (24F/11M) from birth through the 1st-year of life, testing each subject at 5 ages (~1.5, 6, 12, 24 & 52 weeks) with the No-Eye-Contact condition (NEC) of the Human-Intruder-Paradigm accompanied by 18flurodeoxyglucose-Positron Emission Tomography (18FDG-PET). In parallel, one week following each NEC with 18FDG-PET scan, monkeys also underwent resting-state fMRI for seed-based connectivity analyses. With focus on the development of extended amygdala-prefrontal circuitry in relation to preadolescent monkey Anxious Temperament (AT), we characterized the developmental trajectories of threat-related metabolism and resting-state fMRI connectivity in four regions of interest: dorsal amygdala(dA), bed nucleus of the stria terminalis(BST), posterior orbital frontal cortex(pOFC), and dorsal lateral prefrontal cortex(dlPFC). LME analyses demonstrate a general increase in trait-like adaptive threat-related responses as infant monkeys develop, manifesting as within-subject increases in AT from birth to one year. NEC-related metabolism of dA, BST, pOFC, and dlPFC show distinct developmental trajectories, suggesting a shift from relatively equivalent limbic-prefrontal engagement to greater prefrontal engagement during NEC by 1 year. Across this period of significant development, we find associations between BST NEC-related brain metabolism with AT, and age-interactions for pOFC and dlPFC metabolism when predicting AT. Parallel studies demonstrated increased fMRI functional connectivity across development between the dA and regions of the anxiety-related circuit (BST, pOFC, dlPFC). dA-BST and dA-pOFC functional connectivity both demonstrated age-related interactions in relation to AT. Together these data implicate a role of the extended amygdala in the expression of AT across the first year of life, while prefrontal regions show associations with AT later, at 1 year of age. These data provide a developmental framework for understanding risk to develop stress-related psychopathology at behavioral and neural levels in earliest stages of life, and for conceptualizing early-life, neuroscientifically informed, interventions aimed at decreasing risk for stress-related psychopathology.