The effects of tracking in high schools depend in part on the way tracking is organized: To the extent that the structure of tracking varies across schools, tracking's impact on achievement also varies. I examine four structural characteristics of tracking systems: selectivity, electivity, inclusiveness, and scope. I predict that differences in these characteristics lead to variation in between-track inequality (the achievement gap between tracks) and school productivity (average achievement of students in the school), net of the composition of the student body. In addition, I hypothesize that Catholic schools have less inequality between tracks and higher productivity overall than public schools. I test the hypotheses using data from High School and Beyond, a national survey of high schools and their students. The results show that schools vary significantly in the magnitude of track effects on math achievement, and they differ in net average achievement on both math and verbal tests. Schools with more mobility in their tracking systems produce higher math achievement overall. They also have smaller gaps between tracks in both math and verbal achievement when compared to schools with more rigid tracking systems. Moderately inclusive systems also have less between-track inequality in math; and overall school achievement tends to rise in both subjects as inclusiveness increases. The hypotheses about Catholic schools are also supported, especially for math achievement. The way Catholic schools implement tracking partially accounts for their advantages.