The reason for conducting a study of teaching is straightforward: to better understand, and ultimately improve, students' learning, one must examine what happens in the classroom. Although relationships between classroom teaching and learning are complicated, it is well documented that teaching makes a difference in students' learning. Observing that teaching influences students' learning is not the same as claiming that teaching is the sole cause of students' learning, however. Many factors, both inside and outside of school, can affect students' levels of achievement. In particular, eighth-graders' achievement is the culmination of many past and current factors. For these reasons, no direct inferences can or should be made to link descriptions of teaching in the TIMSS 1999 Video Study with students' levels of achievement as documented in TIMSS 1999. Moreover, in most of the participating countries, the videotaped classrooms were not the same ones in which students took the achievement tests.