The purpose of this study was to examine the effects of professional development on teachers' use of sheltered instruction. The participants included 23 teachers from the same elementary school who participated in all components of the study over a span of two years. Teachers demonstrated increased knowledge of sheltered instruction as measured through structured interviews. Teachers also increased their use of sheltered instruction as measured by classroom observations. Results from this study contribute to the research base on the impact of quality professional development in improving teachers' use of instructional techniques, and add to the paucity of research specifically focused on the improvement of instruction for English language learners. (Contains 4 tables.)