Online education is an increasingly popular alternative to traditional classroom- based courses. However, completion rates in online courses are often very low. One explanation for poor performance in online courses is that aspects of the online environment lead students to procrastinate, forget about, or be distracted from coursework. To address student time-management issues, I leverage insights from behavioral economics to design three software tools including (1) a commitment device that allows students to pre-commit to time limits on distracting Internet activities, (2) a reminder tool that is triggered by time spent on distracting websites, and (3) a focusing tool that allows students to block distracting sites when they go to the course website. I test the impact of these tools in a large-scale randomized experiment (n=657) conducted in a massive open online course (MOOC) hosted by Stanford University. Relative to students in the control group, students in the commitment device treatment spend 24% more time working on the course, receive course grades that are 0.29 standard deviations higher, and are 40% more likely to complete the course. In contrast, outcomes for students in the reminder and focusing treatments are not statistically distinguishable from the control. These results suggest that tools designed to address procrastination can have a significant impact on online student performance.