Early childhood adversity, particularly poverty, can be a source of chronic stress that contributes to emotion dysregulation at the start of formal schooling. Children's reactivity to novel challenges in the classroom is associated with externalizing behavior and subsequent difficulties developing academic and social emotional skills (Blair & Raver, 2015; Hackman, Farah & Meaney, 2010). Research grounded in the "science of feeling safe" (Porges, 2011) and attachment theory demonstrates that sensitive, empathic teachers can foster the development of emotion regulation in at-risk children, leading to more adaptive classroom behaviors (Pianta, Belsky, Vandergrift et al., 2008). This study was designed to help explain the associations between student-teacher relationship quality and externalizing behavior, self-regulation and executive function, and then examine if these associations are moderated by poverty. The results of a multilevel analysis highlight significant correlations both "within" and "between" classrooms for closeness/conflict and externalizing behavior and teacher-rated self-regulation. Classroom-level SES was observed to moderate the association of relationship closeness with externalizing behavior. In the discussion of these findings, the needs for more nuanced measures of student stress and teacher sensitivity are examined. Professional development opportunities are also presented for teachers to learn stress management techniques and to build social regulation skills. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]