There is growing consensus among researchers and practitioners that children's socialemotional readiness makes unique contributions to their successful transition to and progress through school. However, many children still begin school ill-prepared for the behavioral demands they will encounter in the classroom. This study examines the joint contributions of social-emotional readiness and classroom experiences to children's social-emotional trajectories in elementary school. Specifically, using a sample of 1292 rural, mostly poor children in Pennsylvania and North Carolina, this study examines: (1) the contributions of children's prosocial behavior, inattention, and conduct problems to their developmental trajectories in these same domains; (2) the effects of classroom management, emotional support, and instructional support in kindergarten through third grade classrooms on children's social-emotional functioning; and (3) the interactive effects of social-emotional readiness and classroom experiences on social-emotional trajectories from kindergarten to grade 3. Data were drawn from a large-scale longitudinal study of children and families living in high-poverty, rural counties in North Carolina and Pennsylvania. This study utilized a longitudinal, non-experimental design. Children's prosocial behavior, inattention, and conduct problems were assessed in the year prior to kindergarten by parent and/or preschool teacher report, and they were assessed again each spring in kindergarten through grade 3 by teacher report. Children's prosocial behavior and conduct problems were assessed by the Strengths and Difficulties Questionnaire, and children's inattention was assessed with items from the ADHD subscale of the Disruptive Behavior Disorders Rating Scale, which was based on Diagnostic and Statistical Manual of Mental Disorders (DSM) diagnostic criteria. Fostering early social-emotional functioning and improving classroom experiences for children are two promising policy levers through which low-income children may be supported as they progress through school. Research suggests that achievement and behavioral gaps between higher- and lower-income children are present at school entry and in fact increase over time. It is thus important to identify the child-level and classroom-level factors that might counter this trend. Although this study is non-experimental, results contribute to the literature on the extent to which social-emotional readiness and certain classroom experiences may reduce skills gaps (i.e., encourage catch-up) among rural, mostly low-income children. Two tables and a figure are appended.