Data about teachers and about the content and methods of teaching are most critical to understanding the conditions of education. Spotty evidence about two recent trends have produced a waft of legislation concerning teacher education, certification, and compensation across the United States. The first consists of data suggesting that the academic ability of those choosing to teach may be declining; the second suggests that the number of prospective teachers will soon be insufficient to meet the demand for new teachers. The data available are inadequate to firmly establish the existence or magnitude of these trends. Projecting teacher supply requires knowledge of at least three sources of potential supply: (1) the number of current teachers expected to remain in teaching; (2) the number of college graduates expected to choose teaching as their initial occupation; and (3) the number of individuals qualified to teach who are currently not teaching but might return to teaching. Data on teaching variables if gathered from teachers sampled in clusters by school and district and, if combined with administrative data on policies and school/student characteristics, would allow analysis of how policy and environmental factors influence teaching practice; how teacher characteristics--including qualifications and experience in the teaching area--influence practice; and how practices change over time. (JAZ)