The authors apply a regression-discontinuity design to identify the causal impact of letter grades on student effort within a course, subsequent credit hours taken, and the probability of majoring in economics. Their methodology addresses key issues in identifying the causal impact of letter grades: correlation with unobservable factors, such as motivation, and direction of causation. They find no evidence that letter grades influence undergraduate students' course-taking behavior or decision to major in economics. They find that, within a course, the first exam letter grade can affect student performance on the second exam.