The present longitudinal study examined how relationships with teachers and peers jointly shape the development of children's behavioral engagement in late elementary school. A sample of 586 children (46% boys; M[subscript age] = 9.26 years at Wave 1) was followed throughout Grades 4, 5, and 6. A multidimensional approach was adopted, distinguishing support and conflict as teacher-child relationship dimensions, and acceptance and popularity as peer relationship dimensions. Additive, moderation, and mediation models were tested. Latent growth curve modeling showed evidence for an additive model in which high initial and increasing levels of teacher support, and high initial levels of peer acceptance, independently reduce the normative declines in children's behavioral engagement. This implies that targeting only 1 relationship in intervention cannot compensate for negative aspects of the other relationship. Teacher conflict only predicted initial levels of behavioral engagement, whereas peer popularity did not predict behavioral engagement (not even in a subsample of children with relatively high levels of relational or physical aggression). However, cross-lagged panel mediation analyses revealed that children who were perceived as more popular in Grade 5 were less engaged in school in Grade 6. Practical implications of these findings are discussed.