Following the argument-based approach to validity, this chapter focuses on the inference about the positive consequences of an academic ESL placement test (EPT) score use on major stakeholders: ESL instructors and ESL students at a university. The assumptions underlying this inference are that the decisions about ESL course placement are justifiable and comprehensible to test stakeholders, that the decisions are beneficial for ESL learners’ improvement of academic English proficiency, and that the decisions are beneficial for ESL learners’ academic achievement at the university. The assumptions were examined using mixed methods that included interviews, pre- and post-course analyses of students’ performances in ESL classes, and an analysis of the predictive power of the EPT results on students’ academic achievement. The results indicated the positive perceptions of the EPT placement decisions as reported by most of the ESL students and instructors, but variation in the degree of improvement was identified in the ESL courses. ESL students’ performance on the EPT predicted their first-year academic achievement at the university to some degree, highlighting the importance of English proficiency in ESL students’ academic success and the potential impact of the EPT placement decisions on students’ English language learning and overall academic performance.