Academic language has been identified as an important focus for instructing students about the quality of written composition they need to be successful in college and career. However, the role of academic language in written composition achievement is not well understood. This study explores the role of academic language skills in students' written composition outcomes. Measures of general academic language at the word, and sentence levels, as well as a reading comprehension measure that included academic text and a word recognition measure were administered to 1316 students in Grade 4 and 1302 students in Grade 8. Students' skill with these four areas were compared to their pass rate on the state accountability test for written composition. Results from a series of general linear mixed models indicate that these four measures predict 65% of the variance in pass rates on a written composition assessment in Grade 4 and 86% in Grade 8. Academic language contributed a small amount of unique, but mostly common variance to distinguishing between proficient and non-proficient writers. Implications related to the unique and common contribution of academic language for improving writing outcomes are discussed.