PROJECT SUMMARY/ABSTRACT Remembering where you left your keys, finding your car in a crowded parking lot, locating items on a map, and navigating through an unfamiliar neighborhood are all tasks that require spatial skills, such as spatial memory, navigation, and reorientation. These spatial skills are adaptive functions that are critical for survival. The hippocampus has been well established as a neural structure that is essential for spatial skills in both animals and human adults. In contrast, relatively little is known about how hippocampal function and structure relates to various spatial skills in children. It is know that, compared to school-aged children and adults, preschool children tend to perform poorly on spatial tasks, suggesting that their hippocampi may not be mature enough to process the demands of such tasks. Unfortunately, these studies have only primarily focused on differences between age groups (e.g., 4-year-olds vs. 6-year-olds) leaving a knowledge gap in our understanding of the individual differences that exists in performance across this span of development, as well as the underlying neural mechanisms which may contribute to this variability. Therefore to address this gap in our knowledge, the current study will examine individual differences in the development of spatial skills and whether these differences can be explained by developmental changes in hippocampal function and structure. Specifically, in the proposed 4-year R01 grant, the project, utilizing an economically- and ethnically/racially-diverse sample of 208 4-, 5-, and 6-year-olds aims to: (1) determine whether individual differences on spatial tasks can be explained by individual differences in hippocampal function by employing a hippocampal-dependent associative learning task (i.e., EBC) using a cross-sectional sample; (2) determine whether individual differences on spatial tasks can be explained by individual differences in hippocampal structure, including subfield volume, neurite density, cortical myelin, and hippocampal-cortical connectivity, measured via structural magnetic resonance imaging using a cross- sectional sample, and; (3) characterize changes in hippocampal function and structure that result in gains in and development of spatial skills using a cohort of 4-year-olds that will be observed longitudinally at 6 years of age. Thus, the proposed grant provides a framework for identifying hippocampal functioning and structure in a developing population not previously studied and for examining the role of the hippocampus in children?s developing spatial competence. In addition to advancing knowledge about the role of the hippocampus in children?s developing spatial skills, results from the proposed research would lay the foundation for future work addressing hippocampal structure and functioning in both typical and atypical (e.g., Williams Syndrome) developing populations, as well as future work examining the cascading cognitive consequences (i.e., spatial working memory; spatial anxiety) of individual differences in hippocampal development.