This article describes an extended program of research in sheltered instruction and the effects on the academic literacy development of English language learners. It also highlights the challenges of scaling up an instructional intervention. The intervention was the Sheltered Instruction Observation Protocol (SIOP) Model, an approach that teaches subject area curriculum to students learning through a second language using techniques that make the content material accessible and also help develop the students' second language skills. Three successive studies looked at teacher change over time and student achievement on standardized assessments and researcher-developed measures. Results of the three studies reveal that students with teachers who were trained in the SIOP Model of sheltered instruction and implemented it with fidelity performed significantly better on assessments of academic language and literacy than students with teachers who were not trained in the model. The article offers guidance for strengthening professional development for teachers so the quality of instruction they deliver to English language learners improves and the students strengthen their English language and academic outcomes. (Contains 4 tables and 3 notes.)