Metadiscourse comprises the linguistic resources used by writers to organize texts, engage readers, project stances towards their texts or readers, or position themselves in relation to readers (e.g. "I suggest," "however," "for example," "perhaps," "it is clear that," "interesting"). Previous research on metadiscourse has primarily focused on highly visible academic genres, such as theses and dissertations, textbooks, and research articles. This dissertation contributes to metadiscourse studies by exploring the use of metadiscourse in an occluded academic genre-- first-year composition teachers' written comments. In addition, researchers in metadiscourse studies have also been interested in correlations between metadiscourse and extralinguistic factors. Previous research has found that genre, discipline, culture, communicative mode, time period, and gender/sex are among those extralinguistic factors that influence the use of metadiscourse. This dissertation contributes to metadiscourse studies by investigating the correlation between metadiscourse and six extralinguistic factors that have not received much attention before, including "course context" ("ENGL 101" or "ENGL 102"), "location of comments" ("end" or "marginal"), "gender," "native language," "years of teaching experience," and "disciplinary background" ("Rhetoric" and "Composition," "Literature," or "Creative Writing"). This study was carried out in a self-built, specialized corpus that consists of 84 samples of written comments from 12 teachers of first-year composition (totaling 30,369 words). The results showed that the frequency of metadiscourse in written comments is exceptionally high with interactional elements being almost twice as common as interactive elements. Through a qualitative analysis, it was found that each subcategory of interpersonal metadiscourse, including engagement markers, endophoric markers, attitude markers, transitions, self-mentions, hedges, boosters, code glosses, frame markers, and evidentials, perform a variety of pragmatic functions in written comments. Using multivariate analysis, this study also assessed how each subcategory of interpersonal metadiscourse varies according to the six extralinguistic factors to a statistically significant degree. The results showed that all the factors appear to influence some use of metadiscourse. "Location of comments" is the most influential factor, and it is correlated with five subcategories, including engagement markers, endophoric markers, transitions, self-mentions, and code glosses. The second most influential factor is "disciplinary background," with such a factor being correlated with three subcategories, including engagement markers, endophoric markers, and boosters. "Course context," "gender," "native language," and "years of teaching experience" all influence two subcategories of metadiscourse. Specifically, "course context" is correlated with attitude markers; "gender" has an impact on the use of endophoric markers and boosters; "native language" influences endophoric markers and engagement markers; and "years of teaching experience" is correlated with endophoric markers and code glosses. Last, previous research has indicated that how teachers express their feedback can affect students' reactions to it and the extent to which they use it in their revisions, which may have an impact on students' writing development. Looking at how teachers use metadiscourse to guide and engage students, to position themselves in relation to students, and to project their stance towards students' writing in written comments, my dissertation also contributes a detailed interpersonal understanding of written comments to scholarship on responses to students' writing in composition studies and provides suggestions for writing program administrators. [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://www.proquest.com/en-US/products/dissertations/individuals.shtml.]