The practice that has come to be known as "tracking" began as a response to the influx of immigrant children into America's schools during the early 20th century. To educate this newly diverse student population, school officials thought it necessary to sort children into different "tracks" based on their ability or past performance. With the new emphasis on preparing every student for college, tracking in its modern form has come to mean grouping students by ability within subjects. In each subject, students are assigned to advanced, regular, or basic courses depending on their past performance. Educators broadly support the practice of tracking in its modern form. Teachers find that tracking facilitates instruction by making it easier to gear lessons to the ability level of the whole class. Parents of high-performing students also favor tracking because research shows that students assigned to high-ability groups make greater gains in achievement. However, in studies published in 1986 and 1999, the author and his colleagues found that students assigned to low-ability groups score lower on standardized tests than if they had been placed in mixed-ability or high-ability groups. That finding lies at the core of a backlash against tracking that began in the 1980s. Critics argued that tracking, especially in practice, created greater learning opportunities for high-performing students at the expense of their lower-performing peers. Tracking's opponents alleged that students in lower tracks often had the weakest teachers in a school, an unchallenging curriculum, few academic role models, and low social status. In this article, the author discusses the detracking movement and its impact on school reforms. (Contains 1 figure.)