The work reported herein was performed pursuant to a grant from the Office of Educational Research and Improvement/Department of Education (OERI/ED). The opinions expressed herein do not necessarily reflect the position, policy or endorsement of the OERI/ED and no official endorsement should be inferred (Grant No. OERI-G-86-1046). This paper was prepared for presentation as part of a National Academy of Sciences symposium entitled “Improving Teacher Quality in Science and Mathematics Education: What Can Research Tell Us?” at the annual meeting of the American Educational Research Association, San Francisco, April 19, 1986. The author would like to acknowledge the helpful comments of Robert Floden, David K. Cohen, and Jere Brophy on earlier drafts.