Social cognitive abilities undergo drastic changes throughout childhood. Theory of mind (ToM), the ability to reason about the mental states of others, is a core social cognitive ability that is crucial for navigating the social world. A majority of prior fMRI research on ToM has characterized the functional response in brain regions that are preferentially recruited to reason about the minds of others in adults. By contrast, a majority of prior developmental research on ToM has used behavioral methods to describe milestones in theory of mind acquisition in early childhood. In my thesis, I draw heavily from these two approaches in order to link them: what is the relationship between the development of functionally selective responses in ToM brain regions, and developmental changes in ToM reasoning in childhood? Experiment 1 includes two longitudinal fMRI studies that test for developmental change and stable individual differences in neural and behavioral measures of ToM, and for predictive relationships between the two measures. Experiment 2 is a large, cross-sectional study that measures the development of the cortical dissociation between brain regions that process minds (the ToM network) and those that process bodies (the Pain Matrix). This experiment additionally provides insight into the neural correlates of passing the false-belief task – the best known developmental milestone in ToM reasoning. Experiment 3 uses a publicly available dataset in order to provide confirmatory evidence for the results described in Experiment 2, and further clarifies the relationship between stimulus-driven functional responses, and inter-region correlations within and between ToM and pain brain regions. Experiment 4 characterizes ToM development, neurally and behaviorally, in children who have experienced delayed access to sign language. This interdisciplinary approach has three broad goals: 1) to characterize the kinds of neural change that support and/or predict behavioral improvements in theory of mind, 2) to gain novel insight into the nature of specific behavioral milestones in social reasoning, and 3) to better understand the impact of experience (e.g., linguistic input) on ToM development, behaviorally and neurally.