Laptop computers have become commonplace in K-12 and college classrooms. With that, educators now face a critical decision. Should they embrace computers and put technology at the center of their instruction? Should they allow students to decide for themselves whether to use computers during class? Or should they ban screens altogether and embrace an unplugged approach? The right way forward is unclear, especially at colleges that pride themselves on connectivity. The personal computer is a powerful tool. It can efficiently store and enhance student work; it can also effectively transport a student's attention away from that work. To determine the impact of laptop usage on student performance, the authors conducted a randomized controlled trial among undergraduate students at the United States Military Academy, widely known by the name of its location in West Point, New York. In the study, they designated who was allowed to use and who was prohibited from using laptops or tablets to take notes in class. They found that allowing any computer usage in the classroom--even with strict limitations--reduces students' average final-exam performance by roughly one-fifth of a standard deviation. This effect is as large as the average difference in exam scores for two students whose cumulative GPAs at the start of the semester differ by 0.17 grade points on a standard 0-4.0 scale. Importantly, these results are from a highly competitive institution where student grades directly influence employment opportunities at graduation--in other words, a school where the incentives to pay attention in class are especially high.