Children that exhibit issues with externalizing behaviors often experience maladaptive outcomes in later life. Externalizing problems during middle childhood (e.g., 6-10 years old) are linked to issues with emotion regulation, which are, in turn, caused by disrupted attention and emotion reactivity to reward. Externalizing problems during this period have also been linked diminished processing of social reward stimuli, suggesting externalizing risk in children may be reflected in contrasting patterns in processing of non-social and social rewards. However, research comparing how differences in affective processing of specific reward content (i.e. social versus non-social) patterns relate to externalizing behavior within normative development is scarce. The following three studies aim to examine neural correlates of affective processing to social and non-social reward stimuli and their relation to externalizing problems in children using a novel, developmentally appropriate image set. The first study (Chapter 2) explores neural processing of different reward categories using event-related potentials (ERPs) in children aged 6-10 years old. Larger ERP responses were associated with early salience processing to object-based (non-social) and intrapersonal reward (social) stimuli. However, larger responses in ERPs associated with sustained processing to these same stimuli categories were associated with greater self-regulation deficits. Gender differences in processing emerged, such that boys showed greater sustained processing to object-based rewards (non-social) than girls. The second study (Chapter 3) examined whether relations children’s ERP responses differed by gender. Greater externalizing problems in boys were associated with greater, indiscriminate processing of stimuli during early stages in neural processing. In contrast, greater externalizing problems in girls were associated with greater sustained processing of both object-based reward and neutral stimuli. The final study (Chapter 4) sought to determine whether ERP responses to distinct forms of social and non-social reward stimuli moderated relations between temperamental traits associated with heightened positive/reward sensitivity (i.e., surgency) and externalizing problems in children. High surgency traits (e.g., impulsivity) in children were associated with greater parent-report of externalizing problems. High surgency was also associated with greater sustained processing of object reward stimuli (non-social) and interpersonal reward stimuli (social). However, these sustained processing patterns did not moderate the relationship between temperament and externalizing problems. Instead, greater externalizing problems were related to greater processing of intrapersonal (social) compared to object-based reward stimuli in high surgency children during early stages in neural processing. Collectively, these results suggest an outsized role for stimulus identity and temporal organization in evaluating early markers for externalizing behavior problems in middle childhood.