To increase opportunities for students to take more advanced math courses in high school, many school districts enroll grade 8 students in Algebra I, a gateway course for advanced math. But students who take Algebra I in grade 8 and skip other math courses, such as grade 8 general math, might miss opportunities to develop the foundational knowledge and skills required for success in advanced math courses. This leaves educators to determine which students are ready for Algebra I in grade 8 and which are not. To inform strategies that address this challenge, this study examined whether student knowledge in five math domains in grade 7 was associated with Algebra I achievement in grade 8. It found that students' scores in all five domains were associated with Algebra I achievement. The "expressions and equations" domain had the strongest association. The association between the "number system" domain and Algebra I achievement was stronger for English learner students than for non English learner students. The associations between the five grade 7 domains and Algebra I achievement did not significantly differ for students who were receiving special education services and those who were not. [For the appendixes, see ED606833; for the study brief, see ED606835; for the study snapshot, see ED606836.]