Low positive affect (i.e., listlessness, anhedonia, and lack of enthusiasm) is a feature of many forms of debilitating psychopathology, including major depression, anxiety, and substance abuse (Epkins & Heckler, 2011; Kendall et al., 2015; Khazanov & Ruscio, 2016; Ramirez et al., 2020; Stanton & Watson, 2015). In childhood, low positive affect is associated with greater instances of self-harm and risk for suicide (Gratz, 2006; Van Beveren et al., 2017). Further, chronically low positive affect in early and middle childhood predicts elevated and stable trajectories of social anxiety and depression (Ghassabian et al., 2014; Griffith et al., 2021; Van Beveren et al., 2016, 2017), which confers harm to children and their families and significant cost to communities (Lynch & Clarke, 2006). In contrast, individuals with high positive affect show higher levels of positive mood states, including happiness, energy, and confidence (Khazanov & Ruscio, 2016; Watson & Naragon-Gainey, 2010, 2014). Positive affect has also been characterized as a marker of psychological health (Gilbert, 2012) and an evolutionarily adaptive trait, indexing greater psychological flexibility, which promotes enhanced social resources (Fredrickson, 1998). Importantly, positive affect has also been studied as important source of resilience, attenuating the impact of stress in adulthood (Ong et al., 2020). However, prior research has shown that child stress, induced by parent and peer conflict, relates to reductions in child positive affect (Bridgett et al., 2013; Ranson & Urichuk, 2008; Russell et al., 2021), with concomitant negative effects on their general mental health and overall well-being (Griffith et al., 2021). Further, while the relationship between stress reductions in positive affect has been established through many cross-sectional studies, less is known about how a global and chronic stressor prospectively disrupts or shapes trajectories of positive affect in children over time, particularly during the transition to adolescence when youth begin to assert their emotional and personal their independence (Christie & Viner, 2005). Since the beginning of 2020, the coronavirus pandemic (COVID-19) has dramatically impacted the lives of children and families, ravaging communities and killing over 4.5 million people worldwide (Johns Hopkins University & Medicine, 2021). The pandemic has had broad implications for child development and mental health (Bartlett et al., 2020; Binagwaho & Senga, 2021; Waller et al., 2021). Many families have had to contend with financial instability and the full-time care and education of their children (Kalenkoski & Pabilonia, 2020; Lancker & Parolin, 2020; Russell et al., 2020). Additionally, many parents have had to adopt physical distancing procedures, eliminating many forms of social support or leisure activities that would otherwise promote positive child development and mental health (Bartlett et al., 2020; Waller et al., 2021). Finally, children and pre-adolescents were forced to adopt online distance-learning or abandon schooling altogether while staying home, prohibiting academic development and all but eliminating in-person peer interactions or socializing opportunities that might otherwise have promoted positive affect and psychological well-being (Lancker & Parolin, 2020). Thus, the COVID-19 pandemic can be characterized as a chronic significant stressor. The direct, detrimental effects of the COVID-19 pandemic on the mental health of children has been well-documented across numerous studies, including elevated depression and anxiety symptoms (Racine et al., 2021), greater parent-child conflict (Russell et al., 2020), higher levels of conduct problems (Waller et al., 2021), and greater misuse of prescription drugs and nicotine (Pelham et al., 2021). However, few studies have leveraged data on child mental health and well-being collected prospectively both prior to and during the pandemic to fully assess the impact of this global stressor on trajectories of child mental health. Importantly, other facets of child temperament can impact their ability to adapt and show resilience during times of stress (Lengua & Long, 2002). In particular, emotion regulation (i.e., the ability to flexibly modulate an emotion or set of emotions (Gross, 2008)) has been found to buffer against anxiety symptoms and depression severity five years later (Berking et al., 2014; Wirtz et al., 2014). On the other hand, child emotion dysregulation has been linked to increases in anxiety symptoms, aggressive behavior, and eating pathology (McLaughlin et al., 2011). Thus, a prospective examination of the links between individual differences in emotion regulation and how children respond to a global stressor, such as the COVID-19 pandemic, can therefore help to clarify etiological pathways to poor mental health trajectories following stress and inform more precise treatment targets for intervention. Recent studies of adults have established that emotion regulation is a key resilience factor that appears to protect against the impact of the COVID-19 pandemic on anxiety (Shilton et al., 2021). However, no study has yet to examine emotion regulation in children as a potential resiliency factor during the COVID-19 pandemic. Thus, to advance knowledge on the direct impact of a global stressor on trajectories of child positive affect and the role of child emotion regulation as a resilience factor, the current proposal will utilize data from the Adolescent Brain and Cognitive Development (ABCD) Study (Feldstein Ewing et al., 2018; Garavan et al., 2018). The ABCD Study has followed participants annually since 2017 (age 9-10 years at recruitment) and administered additional six virtual (online survey) assessments between May 2020 and March 2021, specifically targeted to characterize the impacts of the COVID-19 pandemic. Thus, the current proposal combines longitudinal data from the ongoing ABCD Study that was collected prior to March 11, 2020, when the World Health Organization’s (WHO) declared the novel coronavirus outbreak was a global pandemic, with prospective data collected during the pandemic, to rigorously quantify within-individual changes in child positive emotionality before and during the pandemic (Aim 1). We will then examine associations between COVID-19 related stressors and change in child positive affect during the pandemic, as well as associations with demographic factors such as gender and race given that the pandemic has disproportionality impacted women (Burki, 2020; Connor et al., 2020) and communities of color (Fortuna et al., 2020) (Aim 2). Finally, we will explore the prospective relationship between the hypothesized resilience factor of child emotion regulation (measured via task-based and questionnaire assessments at baseline and immediately preceding the onset of the pandemic) and change in child positive affect during the pandemic (Aim 3).