Background: Parents, teachers, and researchers all share the goal of optimizing students' academic engagement (Handbook of social influences in school contexts: Social-emotional, motivation, and cognitive outcomes, 2016, Routledge, New York, NY). While separate lines of research have demonstrated the importance of high-quality relationships and support from parents and teachers, few studies have examined the collective contributions of adults' warm involvement or the processes by which support from both parents and teachers shapes students' engagement. According to the self-system process model of motivational development, warm involvement from key social partners fosters students' sense of relatedness, competence, and autonomy, (Minnesota Symposium on Child Psychology, Vol. 23: Self processes in development, 1991, University of Chicago Press, Chicago, IL; "Theory and Research in Education," 2009, 7, 133), which subsequently fuels their engagement with academic tasks and challenges ("Journal of Educational Psychology," 2003, 95, 148). Aims: The current study sought to examine whether a sense of relatedness, competence, or autonomy could explain the relation between parents' and teachers' warm involvement and changes in students' academic engagement across a school year. Sample: Data was drawn from 1011 third, fourth, fifth, and sixth graders. Method: Students reported on adult warm involvement, self-system processes, and engagement in the fall and spring of a single school year. Results Structural equation models demonstrated that parent and teacher warm involvement each uniquely, positively, and indirectly predicted changes in students' academic engagement through a combination of students' sense of relatedness, competence, and autonomy, though these patterns differed slightly across adults. Conclusions: Implications for optimizing students' academic engagement are discussed, including the need for intervention efforts focused on both parents and teachers and students' self-system processes.