Childhood overweight and obesity are significant risk factors for elevated cholesterol, blood pressure, and cardiovascular disease throughout the life course. Although parents exert one of the strongest influences on children's health, parent-focused programs to prevent and treat child obesity have had modest success. Parental stress is an understudied factor that may compromise parenting practices related to children's dietary intake and physical activity. However, research linking parental stress and children's obesity risks has suffered from weaknesses including cross-sectional designs, retrospective measures of stress, the failure to consider within-day processes, and a lack of focus on mediating mechanisms-leaving little information to guide the development of prevention and treatment programs that reduce the effects of parental stress on children's obesity risk. To address this gap, we propose a longitudinal study testing a novel conceptual model purporting that the effects of parental stress on children's physical activity and eating behaviors operate through within- day processes that contribute to children's long-term obesity risk in an accumulated manner over time. In the proposed study, 200 working mothers and their 9 to 11 year-old children (N = 400 total) will participate in 6 semi-annual assessments waves across 3 years. Ecological momentary assessment (EMA) and salivary cortisol sampling methods will capture within-day stress processes. Each assessment wave will consist of EMA, salivary cortisol sampling, accelerometry, and 24-hour food intake recalls across 7 days in combination with retrospective paper-and-pencil surveys; and height, weight, and waist circumference assessments in mothers and children. The primary aims are to test hypotheses that (1) greater maternal stress at any given point in the day will predict lower physical activity levels and less healthy eating by children at subsequent points in the day; these within-day effects will be (2) mediated by weight-related parenting practices (e.g., modeling, encouraging, and limiting of children's physical activity and eating) and children's stress, and (3) weaker in households that have rules about children's eating and physical activity, and stronger in households experiencing food insecurity; (4) greater intra-individual variability (i.e., within-person fluctuations) in maternal stress across each 7-day wave will be associated with lower overall physical activity levels and less healthy eating in children across that same 7-day wave; (5) greater maternal stress at baseline will be associated with greater decreases in children's physical activity and health eating, and greater increases in obesity risk over the next 3 years; and (6) change trajectories of maternal stress and children's obesity risk will be inversely relate over the 3 years. The results will inform the development of novel interventions that help working mothers to reduce the negative effects of stress on weight-related parenting practices, and foster family environments that can buffer the effects of maternal stress on children's obesity risk. Given the detrimental health effects of childhood obesity, such information could have significant public health value.