Whether or not to sort students by incoming skills has been a contentious issue. This brief shows that there are potential costs and benefits to both approaches. Even with the same curriculum, the consequences of sorting depend on students' incoming skills and the outcomes being considered. This brief highlights a critical role skill-based sorting plays in shaping students' academic achievement, given a common curriculum for all students. It summarizes findings from a number of prior studies to show what happened after Chicago Public Schools (CPS) introduced two curricular reforms that drastically changed how schools sorted students into classrooms while aiming to teach the same curricular content (Algebra I) to everyone. One policy reduced skill-based sorting and the other increased skill-based sorting. Both policies showed that how schools sort students into classrooms is as important as the content students are exposed to in those classrooms--for students' learning gains, for the grades they receive, and for their pass rates.