Children with obesity (OB), relative to healthy-weight (HW), have reduced hippocampal volume and lower performance on hippocampal-dependent memory tasks. Little is known about whether differences occur across the weight range or in HW children. This three-paper dissertation aimed to 1) replicate and build upon prior volumetric findings in children with OB, relative to HW; 2) evaluate differences in hippocampal volume and tissue biology in youth across the weight range; and 3) examine differences in hippocampal structure and hippocampal- dependent memory in HW children at high-risk (HR; two overweight/OB parents) or low-risk (LR; two HW parents) for obesity. Study 1 used magnetic resonance imaging (MRI) and measures of eating and eating habits in 25 8- to 12-year-old children (13 HW, 12 OB) to examine group differences in hippocampal volume and activation during a functional MRI taste task and explore associations between hippocampal volume and activation and eating/eating behaviors. Children with OB, versus HW, showed reduced left hippocampal volume and greater response to taste in three clusters within the left hippocampus. Moreover, activation within the hippocampus was positively associated with eating and eating behaviors. Study 2 used MRI in 102 adolescents across the weight range to examine the association between standardized BMI (BMIz), hippocampal volume, and tissue biology. BMIz was negatively associated with T2-weighted hippocampal signal intensity in bilateral hippocampi, suggesting differences in tissue biology. Study 3 used MRI, hippocampal-dependent memory tasks, and measures of eating and eating habits in 82 HW children (41 HR, 41 LR) to examine group differences in hippocampal volume and memory and explore whether volume and memory were associated with eating behaviors. HR children, relative to LR, had smaller left hippocampal volumes and lower performance on a hippocampal-dependent word memory task. Moderator models suggested that HR children, relative to LR, may already be showing patterns similar to children with OB. Collectively, these studies demonstrated differences in hippocampal structure and functioning in HW children at HR for OB, relative to LR (Study 1), and in children with OB, relative to HW (Study 2). Moreover, Study 3 findings suggest that, during adolescence, hippocampal differences related to increased weight reflect variations in tissue characteristics.