With the wide introduction of English-medium instruction (EMI) to higher education institutions throughout East Asian countries, many East Asian universities are using English proficiency tests that were not originally designed for this context to make admissions and placement decisions. To support the use of these tests in this new EMI context, it is important to determine (1) whether an international English proficiency test can reflect relevant skills and abilities in university EMI courses, and (2) whether students' scores on the test can be interpreted as indicators of students' performance in university EMI courses. Specifically, this study focuses on the writing section of the TOEFL iBT and examines (1) the comparability of the writing tasks in EMI courses and the TOEFL writing tasks based on task analysis and student and instructor perceptions, (2) the relationships between students' scores on the TOEFL writing section and students' writing performance in EMI courses, (3) the relationship between the quality of the students' writing performance on the TOEFL writing tasks and in course assignments, and (4) students' perceptions of the comparability of the quality of their writing. Participants included 111 undergraduate and graduate students enrolled in one or more EMI courses at Korean universities and 27 instructors. Data analysis included (1) a task analysis of both EMI writing tasks and TOEFL iBT writing tasks, (2) regression analyses on the relationships between the writing test scores and the target language use (TLU) indicators across disciplines, (3) repeated-measure two-way ANOVA examining whether there are differences in the quality of student writing between task types (the TOEFL Integrated task, the TOEFL Independent task, and EMI task) across disciplines, (4) descriptive analyses of students' and instructors' perceptions of the comparability between the TOEFL writing tasks and the EMI writing tasks. Student and instructor interview responses were qualitatively analyzed and used to supplement quantitative findings. The findings of this study have implications for the validity of using an international English proficiency test in EMI contexts with comparisons between different disciplines. In addition, the findings provide insights into language assessment development as well as into designing academic English writing courses in universities. [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.]