Research Findings: This study examines the theory of change of the Chicago School Readiness Project (CSRP), testing a sequence of theory-derived mediating mechanisms that include the quality of teacher-child relationships and children's self-regulation. The CSRP is a multicomponent teacher and classroom-focused intervention, and its cluster-randomized efficacy trial was conducted in 35 Head Start-funded classrooms. Practice or Policy: A series of increasingly complex and conservative structural equation models indicate that the CSRP carries its effects on children's academic and behavioral outcomes through changes in teacher-child relationship quality and children's self regulation. (author abstract)