Despite its importance of academic language, research on academic language is often limited to academic vocabulary and focused on the English language learners. Informed by systemic functional linguistics, this study examined adolescents' use of academic language and the relationships between its use and students' reading ability and their writing quality. Eighty-four ninth-grade students from a public high school in a southeastern state of the United States participated in the study. Three types of analysis were conducted on students' expository essays. First, the essays were rated holistically on a scale of 1-5, with 5 meaning the highest quality. Second, each essay was analyzed for evidence of the following five academic language features: academic vocabulary, expanded noun phrase, nominalization, embedded clause, and lexical density. Third, statistical analyses were conducted to examine the relationship between the use of these academic language features and students' reading ability and writing quality. Linguistic analysis showed that the students used all five academic features in their essays with varying degrees of proficiency. Whereas features such as expanded noun phrase, academic vocabulary, and embedded clause were used with regularity in the students' writing, nominalization was used much less frequently and lexical density was low. Multivariate analysis of variance (MANOVA) indicated a significant relationship between the students' use of academic language and their reading ability, defined by their scores on a state reading achievement test. Compared to students of lower reading abilities, students of higher reading abilities used significantly more nominalizations and showed higher lexical density in their writing. However, no significant differences were found between the reading groups in the use of academic vocabulary, embedded clause, and expanded noun phrase. Analysis of variance (ANOVA) showed that higher-quality essays scored significantly higher than lower-quality essays on expanded noun phrase, nominalization, and lexical density, but not on academic vocabulary and embedded clause. Backward stepwise regression analysis revealed that nominalization predicted 43% of the variances in writing quality and that academic vocabulary had a negative relationship with writing quality. These findings suggested that the relationships between academic language use and reading ability and writing quality are complex and warrant further exploration. (Full text of this dissertation may be available via the University of Florida Libraries web site. Please check http://www.uflib.ufl.edu/etd.html ) [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]