In this article, the authors describe a field experiment in the classroom where principles of micro-economics students are randomly assigned into homework-required and not-required groups. The authors find that homework plays an important role in student learning, especially so for students who initially perform poorly in the course. Students in the homework-required group have higher retention rates, higher test scores (5 to 6 percent), more good grades (Bs), and lower failure rates. The authors also study the relationship between endogenous homework submission and test performance using instrumental variable estimation and find that homework submission has a large positive effect on test performance. (Contains 5 tables and 12 notes.)