The goal of ESL courses offered in American universities is to prepare international students for communication in other courses. This chapter investigates the degree of correspondence in the writing tasks used for assessment in an ESL course and those in other courses. The argument-based validity study focused on extrapolation, the degree of correspondence between performance on the assessment tasks and performance in the context of interest to scores users. Accordingly, this study compared course syllabi, assignment sheets, and students’ written assignments as well as analyzing instructor interview data and student survey responses. The results suggest that the ESL writing and disciplinary course assignments differ with regard to genre, topic, information sources, purpose, rhetorical functions, and length despite their similarity of target audience. In addition, interview data show that students tended to notice more differences than similarities in assignments in the ESL and other courses. The findings do not support the extrapolation inference in the interpretation/use argument for the assessed writing in the ESL courses. Instead, findings point to suggestions for revision to these writing assignments in the ESL academic writing course.