The magnitude of variation in the quality of teachers, even within each school, is startling. Teachers who work in a given school, and therefore teach students with similar demographic characteristics, can be responsible for increases in math and reading levels that range from a low of one-half year to a high of one and a half years of learning each academic year. But while most parents are able to distinguish a good teacher from a bad one, few have any idea what difference it makes in the lives of their children. And researchers do not help, tending to talk in terms of standard deviations of achievement and effect sizes, phrases that simply have no meaning outside of the rarefied world of research. In this article, the author translates the researchers' shorthand into concepts that might be more readily understood: the impact of teachers on the earnings of individuals and on the future of the economy as a whole. (Contains 2 figures.)