Equality of educational opportunity is threatened by long-standing gaps in student achievement by race, gender, and student poverty, as well as by school sector and school poverty. The true magnitude of these gaps cannot be understood, however, unless these factors are considered simultaneously. While accounting for the effects of demographic characteristics, this article focuses on the effects of school sector and school poverty on gains in academic achievement. Analyses from a longitudinal study of public and Catholic middle school students in and around the city of Chicago show that neither the public nor Catholic sector has a consistent advantage in increasing student achievement in sixth and eighth grade reading and mathematics. School poverty has a deleterious effect on student achievement, although this effect is considerably mitigated for students in Catholic schools. (Contains 10 tables.)