The ACT is an educational achievement test that measures the typical content and skills learned from college preparatory curricula. Consequently, the ACT can be used not only to predict college success, but also to provide direct feedback to high school teachers about the effectiveness of their teaching. ACT results also assist teachers in adapting instruction to individual students, and assist high school administrators in evaluating their educational programs. Perhaps most important is that ACT results show a student exactly what skills he or she needs to improve upon. In short, the most profound advantage of the ACT is not its predictive validity, but rather the information it provides to intervene and to improve students' chances to do well in college. Since the ACT is a curriculum-based test, how students perform on it depends to a large extent on the courses they take and how well they master their course work. National ACT trends show that students who take a college preparatory core curriculum in high school typically score 2-3 points higher (on a 1-36 point scale) on the ACT subject area tests (in English, mathematics, reading, and science) and the Composite than those who do not. Current ACT research shows that students' academic preparedness for college, as measured by their ACT scores, is the result of the specific courses they take, not just the numbers of years they take. Moreover, taking more advanced courses increases the likelihood that a student will be prepared. These findings were obtained using the high school GPAs, ACT scores, grade level at the time of testing, and high school course work taken for over 1 million ACT-tested 2004 high school graduates. This information was used to show the effects of taking particular course sequences on ACT English, Mathematics, and Science scores, regardless of student' prior achievement and grade level. (Contains 1 figure.)