Dual language learners make up an increasing share of preschool students, but they often perform worse than monolingual students on assessments measuring school achievement. This study compares Head Start classrooms implementing either the dual language model or the English with home language support model. The author examines how the models affect gains in English or Spanish oral proficiency over a school year and how classroom organization and quality affect potential proficiency gains. Students in dual language classrooms showed significantly greater average gains from pretest to posttest in English oral proficiency and Spanish oral proficiency than did students in classrooms using the English with home language support model. The difference was even more pronounced among classrooms with low organization. Findings highlight the need for professional development on language model use to ensure consistency in delivery. [This work was funded through the Young Scholars Award.]