Project Summary School readiness skills prior to entering elementary school are crucial for later academic success and, importantly, these skills may act as a protective factor for low-income students as they transition into Kindergarten and continue through elementary school. However, extant studies of school readiness before elementary school have largely defined and measured this construct in narrow and potentially inaccurate ways. For the most part, studies have focused on cognitive assessments, ignoring important social-emotional characteristics that are required to perform well in the classroom. For studies that have taken these factors into account, this is often assessed by parent or teacher report or in the home or lab settings, which ignore the way in which the demands of the classroom environment may affect functioning. In addition, these assessments are primarily behavioral; very few studies have attempted to understand the interplay of children’s psychophysiological functioning and their classroom experiences. Indeed, there is a substantial gap in our knowledge regarding how psychophysiological functioning may impact students’ school readiness and early school success; this is particularly salient if we seek to improve academic outcomes for low-income children through early education. To address this gap, a diverse sample of families will be recruited for the proposed short-term longitudinal study. We will enroll 270 children from approximately 30 classrooms prior to their final year of preschool (preK). Across preK, children will participate in several data collection visits including fall and spring school readiness assessments. In addition, on two occasions across the school year, we will observe children in the classroom while also collecting cardiac data. Finally, at the end of Kindergarten, several assessments of school success will be conducted. These data will be used to examine how teacher behavior and children’s respiratory sinus arrhythmia (RSA), a key indicator of psychophysiological regulation and function, work together to predict school readiness and early school success. This study will be the first to examine school readiness in a sample of low-income children that includes conducting naturalistic observations (and cardiac monitoring) of children and their teachers in the preK classroom at two timepoints, with a focus on the parasympathetic nervous system (PNS) as a mechanism that may play a critical role the way in which children respond differently to teacher sensitivity and behavior, and subsequently predict school readiness and school success in Kindergarten.