PROJECT SUMMARY Many neuropsychiatric diseases, such as depression, are characterized by deficits in memory and affective processing—processes that are mediated in part by the hippocampus. Atypical cortico-hippocampal resting state functional connectivity (RSFC) has been associated with poor memory performance and depression. RSFC relies on the observation that spatially separated but functionally related regions (i.e., networks) exhibit correlated brain activity, making it an ideal method for a systems-level interrogation of brain activity. Childhood is a period of great plasticity and vulnerability, when many psychopathologies first manifest. Disruptions in the development of cortico-hippocampal connectivity are thought to contribute to developmental psychopathology. Throughout development, there is continued development of hippocampally-mediated cognition and emotional regulation. The extent to which hippocampal RSFC is mature by middle childhood is unknown. To understand how atypical hippocampal FC may contribute to developmental psychopathology, we must better understand hippocampal functional organization during development and its relationship to behavior. Prior studies have utilized group-averaging to demonstrate the hippocampus is functionally connected to the default mode network. However, recent efforts have found that precision functional mapping (PFM), using large quantities of data per participant, reveals novel individual-specific features of network organization in the adult hippocampus. Namely, the anterior hippocampus is preferentially functionally correlated to the default mode network (DMN) and the posterior hippocampus to the parietal memory network (PMN). The current proposal aims to test whether the hippocampus in individual children also exhibits a DMN-PMN dichotomy and if individual differences in hippocampal RSFC can predict behavior. To address these questions, we will collect a highly-sampled pediatric dataset (n=10; 9-10 yrs) as part of the PFM-pediatric Study and also utilize a separate independent, large-sample size dataset (Adolescent Brain Cognitive Development [ABCD]; n=11,874; 9-10yrs). I hypothesize that since network architecture is largely in place by middle childhood, there will be a DMN-PMN segmentation of the hippocampus. I will also utilize the ABCD’s measures of episodic memory, attentional control and depression severity to predict associations with hippocampal RSFC. Understanding the basic organization of the hippocampus with individual specificity will pave the way for the development of more precise, patient-specific models of hippocampus-related pathologies, which may be crucial for early diagnosis and treatment. The current proposal serves as a foundational first step to future investigations into the behavioral consequences of individual differences in hippocampal network organization and the development of psychopathology.