PROJECT SUMMARY We propose to investigate mask wearing and child development in the context of the ongoing Baby’s First Years Study (BFY; “Household Income and Child Development in the First Three Years of Life”; R01HD087384). We will gather information about the child’s own mask-wearing behavior, as well as the mask wearing of other people in their everyday environment, around the time of the child’s third birthday. This information will be used to i) characterize mask-wearing behavior for children and caregivers in a geographically diverse sample of low-income mothers and their children and ii) relate mask wearing in the child’s environment (e.g., by their mother, peers, caregivers, and relatives) as well as the child’s own mask- wearing behavior, to the child’s language, socioemotional, and neural development.