It has been 25 years since the publication of J. B. Carroll's (1963) seminal work on the model of school learning (MSL) and its subsequent identification by B. S. Bloom (1968) as the theoretical basis underlying mastery learning. Carroll's model of school learning is a theoretical formulation asserting that the degree of learning attained on a given task is a function of the time spent by a student on the task divided by the time needed by the student for the mastery of that task. Bloom's mastery learning theory encompasses: (1) a set of optimistic assumptions regarding the capability of students to learn the contents of a course/unit to a high degree of proficiency provided that learning conditions are maximized; and (2) an array of instruction-based functions or purposes served by the preparatory, delivery, and evaluative tasks of diagnostic-prescriptive teaching. This paper provides a 25-year retrospective view of the foundational role assumed by the variables and operations of Carroll's MSL in directing mastery learning efforts. More specifically, attention is given to the strategic impact of Carroll's conceptualizations of aptitude, quality of instruction, and opportunity to learn on Bloom's optimism regarding the capability of students to learn and his diagnostic-prescriptive teaching regimen. (Author/SLD)