As state and national policymakers look for ways to improve the rigor of the high school curriculum and enhance student readiness for college, many are turning their attention to increasing course requirements in core academic high school subjects. Existing research shows that students who take high-level course sequences learn more in high school and are more likely to attend and perform better in college than students who do not take these classes. Yet, despite the appeal of mandatory curriculum policies, surprisingly little is known about whether changing course requirements will necessarily lead to improved outcomes for students. This is because the previous studies cited by many in the policy and reform communities do not fully correct for what researchers call selection bias: that is, the students who choose to take high-level classes are often the most motivated and high-achieving students in their schools, and the schools that offer advanced courses to students are those with the capacity to teach them and are often college-oriented in other ways. To inform state and district curriculum policies, and to address some of these limitations of the previous research, research teams at the Consortium on Chicago School Research (CCSR) at the University of Chicago and at the University of Michigan spent the last three years examining a 1997 Chicago Public Schools (CPS) effort to implement a version of the mandatory college-preparatory curriculum. The 1997 CPS policy change ended remedial classes and required college-preparatory coursework for all students in four subject areas: English, math, science, and social studies. This policy brief describes the key findings from this study of the 1997 reform and highlights its implications for ongoing state and national policy deliberations. (Contains 3 figures and 13 footnotes.) [This policy brief was drawn from the following study: Allensworth, Elaine M., Takako Nomi, Nicholas Montgomery, Valerie E. Lee. 2009. College Preparatory Curriculum for All: Academic Consequences of Requiring Algebra and English I for Ninth Graders in Chicago. "Educational Evaluation and Policy Analysis," 31 (4), pages 367-391 (EJ866927).]