Few individuals in the history of education have had greater impact on education policy and practice than Benjamin S. Bloom. During a career that spanned more than 5 decades, his research and writing guided the development of many educational programs and provided insights into the untapped potential of educators to have all students learn well. Bloom's contributions to education began during his years in the Office of the Board of Examiners at the University of Chicago, where he worked from 1940 to 1959. Much of his work at this time focused on the relationships among methods of instruction, educational outcomes, and measurement of those outcomes. This work led to his first book in 1950 and eventually to the work for which he is best known, the "Taxonomy of Educational Objectives, Handbook I: The Cognitive Domain." In 1959 Bloom spent a year at the Center for Advanced Study in Behavioral Sciences at Stanford University. This year marked a shift in high research and writing, as Bloom began to concentrate on problems in learning, rather than problems in testing, measurement, and evaluation. Bloom's most notable contribution to teaching and learning was his work in developing the theory and practice of mastery learning. Mastery learning was developed as a way for teachers to provide higher quality and more appropriate instruction for their students. Mastery learning depends on feedback, correctives, and enrichments, combined with another essential element of mastery learning, congruence among instructional components. Mastery learning is not an educational panacea, but careful attention to the essential elements of mastery learning will allow educators at all levels to make great strides toward the goal of all children learning excellently. (Contains 44 references.) (SLD)