Use of research-based practices for improving content area literacy of English learners is important both because of their growing numbers and their overall poor academic performance. However, how do we ensure that teachers are implementing proven methods with fidelity? This study shows the direct relationship between teachers' level of implementation of such practices on student achievement. Specifically, we report findings from research through the Center for Research on the Educational Achievement and Teaching of English Language Learners (CREATE). In the study, teachers learned and implemented the Sheltered Instruction Observation Protocol (SIOP) Model of instruction to improve their students' language and content literacy development. The degree to which teachers implemented the model with fidelity resulted in relative improvement in student performance, underscoring the importance of teacher implementation of proven practices. (Contains 3 figures and 1 table.)