This study focused on the joint effects of teachers and peer groups as predictors of change in students' engagement during the first year of middle school, when the importance of peer relationships normatively increases and the quality of teacher-student relationships typically declines. To explore cumulative and contextualized joint effects, the study utilized 3 sources of information about an entire cohort of 366 sixth graders in a small town: Peer groups were identified using sociocognitive mapping; students reported on teacher involvement; and teachers reported on each student's engagement. Consistent with models of cumulative effects, peer group engagement and teacher involvement each uniquely predicted changes in students' engagement. Consistent with contextualized models suggesting differential susceptibility, peer group engagement was a more pronounced predictor of changes in engagement for students who experienced relatively low involvement from teachers. These peer effects were positive or negative depending on the engagement versus disaffection of each student's peer group. Person-centered analyses also revealed cumulative and contextualized effects. Most engaged were students who experienced support from both social partners; steepest engagement declines were found when students affiliated with disaffected peers and experienced teachers as relatively uninvolved. High teacher involvement partially protected students from the motivational costs of affiliating with disaffected peers, and belonging to engaged peer groups partially buffered students' engagement from the effects of low teacher involvement. These findings suggest that, although peer groups and teachers are each important individually, a complete understanding of their contributions to students' engagement requires the examination of their joint effects.