This study examines tracking--the practice of grouping students into separate classes or courses based on their prior academic achievement--at the middle-school level, and the percentage of high-achieving students in tracked and untracked schools. It focuses on Massachusetts, a leader in "reforming" tracking, and the changes that have occurred there over eighteen years (1991-2009). Specifically, the study addresses the following: (1) How has tracking changed since the 1990s? (2) Why have some schools detracked while others are resisting such reform? and (3) Are there differences in the number of high-achieving students associated with tracked and untracked schools? Specifically, do tracked and untracked schools produce similar percentages of students reaching the "advanced" level on the Massachusetts Comprehensive Assessment System (MCAS)? Key findings include: (1) Tremendous change has occurred in tracking since the 1990s; (2) Several factors influence tracking policy; (3) Detracking is more prevalent in urban, high poverty schools; and (4) Detracking carries risks for high-achieving students. A Multivariate Model of Influence on Tracking Policy is appended. (Contains 16 tables, 2 figures, and 42 endnotes.) [Foreword by Chester E. Finn, Jr. and Amber M. Winkler.]