Research Findings: This study used the Theoretical Domains Framework (TDF) to examine barriers to teachers’ implementation of a supplemental academic language curricula. Despite high satisfaction with this scripted curriculum, three past studies indicated heterogeneity in teachers’ fidelity of implementing the curriculum as well as difficulty going off script. Thus, our goal was to identify barriers to fidelity of implementation and map these onto possible behavior change techniques. Participants included 175 teachers from pre-kindergarten and kindergarten classrooms. We used mixed-method approaches that included surveys, focus groups, and interviews with teachers and their coaches. The most salient barriers to fidelitious implementation were: competing priorities for instructional time as well as limited teacher knowledge and skills. For some teachers, other barriers included difficulty changing habits, challenges with memory and attention processes, or lack of fit when the curriculum was used with populations beyond the designed scope. Practice or Policy: To understand why teachers may not consistently implement evidence-based curricula with fidelity, we explain steps other education researchers can use to apply the TDF and insights from the field of implementation sciences. We provide sample TDF survey questions and suggestions to help educators and researchers systematically revise the theory of change for curricular interventions. [For the grantee submission, see ED612067.]