This report is about conceptual and methodological issues that arise when educational researchers use data from large-scale, survey research studies to investigate teacher effects on student achievement. In the report, we illustrate these issues by reporting on a series of analyses we conducted using data from Prospects: The Congressionally Mandated Study of Educational Opportunity. This large-scale, survey research effort gathered a rich store of data on instructional processes and student achievement in a large sample of U.S. elementary schools during the early 1990s as part of the federal government's evaluation of the Title I program. We use data from Prospects to estimate the "overall" size of teacher effects on student achievement and to test some specific hypotheses about why such effects occur. On the basis of these analyses, we draw some substantive conclusions about the magnitude and sources of teacher effects on student achievement and suggest some ways that survey-based research on teaching can be improved.