The goal of this study was to assess the effectiveness of an intervention--Quality English and Science Teaching (QuEST)--designed to develop the science knowledge and academic language of middle grades English language learners studying science in their second language and their English-proficient classmates. Both English language learners and English-proficient students are incorporated in this study design, and the authors have tested explicitly for interactions of language status with experimental and traditional forms of instruction. Overall, it appears that the implementation of QuEST improves the quality of teachers' science instruction and raises student performance on curriculum based measures of Vocabulary and Science. Project QuEST differed from the practices in the control classrooms by making alterations to accommodate the needs of ELLs and build on their strengths. Consistent with the literature on effective L2 instruction, the content was made clear to students through the use of visuals, modeling, and ongoing discussion. Additionally, students' English oral proficiency was developed in the context of science instruction through explicit vocabulary instruction, guided reading, and partnering with classmates who were more English proficient. (Contains 2 footnotes.)