This chapter focuses on the rise of the age-graded classroom as the preferred school organization method. Before exploring the characteristics and ambiguous meanings surrounding the terms “grade” and “gradation,” this section situates the study within the context of US-American urban education. It then discusses the precursors to gradation, more specifically individualized collective instruction, the monitorial method, and the beginnings of gradation that emerged in the wake of the common school movement. This historical perspective is followed by a detailed discussion of gradation and the different arguments that have been employed to explain its institutionalization. Despite being widely implemented in most school systems, age-grading has faced persistent scrutiny within the educational discourse. The final part of the chapter delves into specific critiques of this organizational practice.