Abstract Reward-related brain function is consistently linked to greater motivation, pleasure, and goal-directed behavior and lower risk for depression across the lifespan. Healthy neural reward response supports socioemotional development, particularly during the preschool years as self-regulatory skills and child reward-related brain function are rapidly developing in the context of the caregiving environment. Maternal socialization of positive emotion is one important influence on early reward circuitry development. Mothers with depression are more likely to discourage (and less likely to encourage) child positive emotions compared to healthy mothers, which may contribute to early neural reward alterations in their children. Characterizing mechanisms of influence of maternal socialization on child neural reward response and positive emotion during the preschool years is a critical window of opportunity when parents have a large influence on child socioemotional development. Importantly, maternal behavior is amenable to change by training parents on emotion coaching, and these maternal behavior changes may result in direct and immediate changes in child neural reward function. Thus, the overarching aim of this proposal is to use a mechanistic trial design to experimentally test the hypothesis that maternal encouragement of child positive emotion will lead to in-vivo increases in child neural reward response. Event-related potentials (ERPs) are uniquely suited to non-invasively assess in-vivo, fast-occurring changes in child reward response during parent-child interactions, including reward positivity (RewP), and the late positive potential (LPP) amplitudes. Toward this aim, we will randomize 180 mothers with clinically significant depression symptoms and their 4- to 6-year-old children (50% female) to receive either 3 control sessions or 3 positive emotion coaching modules from the Parent-Child Interaction Therapy-Emotion Development (PCIT-ED, which trains mothers on how to encourage positive emotion in their young children. Children will complete reward tasks at pre- and post-coaching, while neural reward response is assessed via ERP (RewP and LPP) with their mothers present allowing for in-vivo assessment of maternal behavior. At both timepoints, we will assess child neural reward response and mothers’ self-reported maternal socialization behaviors. Understanding how disrupted neural reward responding develops in the preschool years is critical for the promotion of child emotional wellness and can be directly used to develop preventive interventions tailored to preschool age children at familial risk for depression.