States and districts have expressed interest in administering the ACT® to 10th-grade students. Given that the ACT was designed to be administered in the spring of 11th grade or fall of 12th grade, the appropriateness of this use should be evaluated. As such, the focus of this paper is to summarize empirical evidence evaluating the use of the ACT as a measure of college readiness for 10th graders. In alignment with a Standards for Educational and Psychological Testing, empirical evidence related to five sources of validity evidence (response processes, internal structure, content, relation to other variables, and consequences) are summarized. As compared to 11th-grade test administrations, the results indicate that when the ACT is administered to all 10th graders: (1) Students are similarly motivated (response process); (2) Scores are only slightly less reliable (internal structure); and (3) ACT scores and test completion rates are predictably lower for 10th graders relative to 11th graders. Additionally, the results indicate that: (1) The content of the test is aligned to college readiness standards based on what students learn in high school and need to know to succeed in college (content); (2) How students perform on the ACT in 10th grade is comparable to other 10th-grade measures (i.e., PreACT®) of college readiness; and (3) ACT scores from 10th grade are predictive of high school grades and 11th grade ACT scores (relation to other variables). Unlike other 10th grade tests, the ACT provides a college-reportable score and greater test security (relative to the PreACT test). Collectively, the evidence supports the use of the ACT as a measure of college readiness or academic preparation level for 10th-grade students.