Despite the current theoretical momentum for the importance of academic English and the acknowledgment that academic materials increase in complexity through the grades, little empirical attention has been devoted to the role of academic English in academic achievement. This study examined the amount of variance in academic achievement explained by academic word knowledge for diverse middle school students. A linguistically and socioeconomically diverse sample of grade 7 and 8 students (N = 339) was administered measures of overall breadth of vocabulary knowledge and general (i.e., cross-discipline) academic word knowledge, and the explanation of variance in standardized academic achievement tests across 4 disciplines was explored. For the entire sample, knowledge of general academic words explained a considerable and significant amount of variance in academic achievement across 4 disciplines. Findings lend empirical support to current calls for providing academic language support for early adolescents from non-native English speaking and low-socioeconomic backgrounds. (Contains 2 figures and 4 tables.)