In recent years, a consensus has emerged among researchers that teacher quality matters enormously for student performance. Students taught by more-effective teachers learn substantially more over the course of the year than students taught by less-effective teachers. Yet little is known about what makes for a more-effective teacher. Most research on teacher effectiveness has focused on teacher attributes, finding that readily measurable characteristics such as experience, certification, and graduate degrees generally have little impact on student achievement. Relatively few rigorous studies look inside the classroom to see what kinds of teaching styles are the most effective. The authors tackle this underexplored area by investigating the relative effects of two teacher practices--lecture-style presentations and in-class problem solving--on the achievement of middle-school students in math and science. Contrary to contemporary pedagogical thinking, the authors find that students score higher on standardized tests in the subject in which their teachers spent more time on lecture-style presentations than in the subject in which the teacher devoted more time to problem-solving activities. (Contains 3 figures.)