In a 1984 essay, Benjamin Bloom, an educational psychologist at the University of Chicago, asserted that tutoring offered "the best learning conditions we can devise" and that tutors could raise student achievement by two full standard deviations--or, in statistical parlance, two "sigmas." The influence of Bloom's two-sigma essay reached well beyond the scholarly literature. As computing and telecommunication revolutions advanced, visionaries repeatedly highlighted the potential of technology in response to Bloom's argument that the major challenge facing education was to devise more economical interventions that could approach the benefits of tutoring. This article explores Bloom's claims and how realistic it is to expect any kind of tutoring to improve student achievement by two standard deviations.