Will college students who set goals for themselves work harder and perform better? In theory, setting goals can help time-inconsistent students to mitigate their self-control problem. In practice, there is little credible evidence on the causal effects of goal setting for college students. We report the results of two field experiments that involved almost four thousand college students in total. One experiment asked treated students to set goals for performance in the course; the other asked treated students to set goals for a particular task (completing online practice exams). We find that performance-based goals had no discernible impact on course performance. In contrast, task-based goals had large and robust positive effects on the level of task completion, and task-based goals also increased course performance. Further empirical analysis indicates that the increase in task completion induced by setting task-based goals caused the increase in course performance. We also find that task-based goals were more effective for male students. We develop new theory that reinforces our empirical results by suggesting two key reasons why task-based goals might be more effective than performance-based goals: overconfidence and uncertainty about performance. Since task-based goal setting is low-cost, scaleable and logistically simple, we conclude that our findings have important implications for educational practice and future research.