Abstract: Our capacity to understand ourselves and others, both in terms of stable personality traits and changing mental or emotional states, is critical to typical development. Relatively little is known, however, about the neural systems that support these important social cognitive processes in children or adolescents. Drawing from the first wave of a longitudinal study of brain development, components of these two complementary aspects of mentalizing were examined using functional magnetic resonance imaging in 90 children (aged 9-10 years). Study 1A demonstrated that children utilized medial prefrontal cortex (MPFC), a brain region implicated in self-reflection, significantly more than did adults when retrieving trait knowledge about the self—suggesting preadolescents may be less self-focused in the absence of external task demands. However, study 1B showed that in highly positive self-concept domains, self-knowledge retrieval was associated with activity in the amygdala, precuneus, and dorsolateral prefrontal cortex, which may indicate the very beginnings of a shift from more explicit, evidence-based systems to relatively intuitive, automatic affective ones that can be directly linked to motivation. This highlights a potential mechanism by which self-concepts could influence developmental outcomes. Study 2A provided evidence of strong correlations between activity in the mirror neuron system (MNS), a network for shared representations of our own and others' emotions, and empathy as well as interpersonal competence—suggesting that shared neural representations might facilitate more effective mentalizing. Finally, study 2B explored moderators of activity in the MNS and several additional systems for shared neural representations of social behavior (including MPFC), and found that gender group membership, explicit gender bias, and levels of sex steroid hormones moderated the brain's response to gender ingroup and outgroup members. Specifically, group membership alone was associated with increases in amygdalar activity to ingroup members; greater gender bias was associated with increased activity in MPFC and the MNS to ingroup members; and higher estrogen was associated with increased activity in ventral striatum while girls in early puberty viewed boys. Taken together, these results provide a strong foundation for future studies of the neural systems supporting self and social perception during the transition from childhood to adolescence.