This study explores the relationship between teacher knowledge and student learning in the area of mathematics by developing and evaluating an innovative approach to assessing teacher knowledge. This approach is based on teachers' analyses of classroom video clips. Teachers watched 13 video clips of classroom instruction and then provided written comments on the interactions of the teacher, students, and content. The quality of teachers' analyses, coded using an objective rubric, are shown to be reliable and valid, relating both to another widely used measure of teacher knowledge and to teachers' own students' learning (from pre- to posttest). (Contains 1 table and 3 notes.)