We conducted a randomized field trial to test an academic vocabulary intervention designed to bolster the language and literacy skills of linguistically diverse sixth-grade students (N = 2082; n = 1469 from a home where English is not the primary language), many demonstrating low achievement, enrolled in 14 urban middle schools. The 20-week classroom-based intervention improved students' vocabulary knowledge, morphological awareness skills, and comprehension of expository texts that included academic words taught, as well as their performance on a standardized measure of written language skills. The effects were generally larger for students whose primary home language is not English and for those students who began the intervention with underdeveloped vocabulary knowledge. [This paper was published in "American Educational Research Journal" v51 p1159-1194 2014 (EJ1045653).]