The increase in language minority students in U.S. schools has drawn attention to instructional models designed to assist these students. It is critical that literacy educators understand the potential of these models for literacy achievement. This study was designed to examine reading achievement of elementary English language learners in classrooms where teachers implemented a popular sheltered instruction model called SIOP (Echevarria, Vogt, & Short, 2004) compared with students of teachers who have not received instruction in the model. Through a mixed methods study linking professional development, teacher learning, and student achievement, researchers found significant differences in classrooms where the model was well-implemented. However, the differences come with qualifications, and the findings are critiqued in light of sociocultural theory and studies on professional development. (Contains 1 table and 2 figures.)