Scores from standardized tests are common criteria for admission to a four-year college or university, and most require, recommend, or consider test scores as part of the admissions process. To help improve their chances of college admission, many high school students turn to test-preparation programs. We used an experimental design to evaluate the effectiveness of an online test-prep course to improve test scores. Specifically, given the current climate around test preparation and the call for more and better research evidence documenting the impact of test preparation, this research study sought to provide representative, relevant, and rigorous research on the use of ACT® Online Prep (AOP).