The RULER Approach to Social and Emotional Learning ("RULER") is designed to improve the quality of classroom interactions through professional development and classroom curricula that infuse emotional literacy instruction into teaching-learning interactions. Its theory of change specifies that RULER first shifts the emotional qualities of classrooms, which are then followed, over time, by improvements in classroom organization and instructional support. A 2-year, cluster randomized controlled trial was conducted to test hypotheses derived from this theory. Sixty-two urban schools either integrated RULER into fifth- and sixth-grade English language arts (ELA) classrooms or served as comparison schools, using their standard ELA curriculum only. Results from multilevel modeling with baseline adjustments and structural equation modeling support RULER's theory of change. Compared to classrooms in comparison schools, classrooms in RULER schools exhibited greater emotional support, better classroom organization, and more instructional support at the end of the second year of program delivery. Improvements in classroom organization and instructional support at the end of Year 2 were partially explained by RULER's impacts on classroom emotional support at the end of Year 1. These findings highlight the important contribution of emotional literacy training and development in creating engaging, empowering, and productive learning environments.