Researchers have found that students who attend Catholic high schools tend to outperform public high school students on standardized tests of achievement. Although many aspects of this finding have been examined in subsequent research, little attention has been paid to the issue of how ability grouping affects achievement across school sectors. A nearly universal practice in middle and secondary schools, ability grouping works to channel learning opportunities to students. The authors trace the history of ability grouping and review the findings regarding ability group effects, the assignment process, and mobility across groups in each school sector. Their analyses suggest that the way ability grouping is implemented in Catholic schools contributes to the Catholic school advantage in achievement. (Contains 1 table.)