This article describes a study examining the effects of Sheltered Instruction Observation Protocol (SIOP) model instruction on the academic language performance of middle and high school English language learners. The SIOP model is an approach for teaching content curriculum to students learning through a new language. Teachers employ techniques that make the content concepts accessible and also develop students' skills in the new language. Using a quasi-experimental design, the research was conducted in content area and English as a second language classes in two districts in northern New Jersey over 2 years. The analysis presents student achievement data from state-mandated language proficiency tests in the final year of the intervention, after most of the treatment teachers had completed their professional development in the SIOP model. There were statistically significant differences in the average mean scores in favor of the treatment student group on Writing, Oral Language, and Total English scores of the IDEA Language Proficiency Tests with small to medium effect sizes. The results from this study show that the SIOP model offers a promising approach to professional development that can improve the quality of instruction to English language learners and increase their English language achievement. (Contains 5 figures, 8 tables and 1 footnote.)