It is generally accepted in the literature that writing assignments, even short ones, increase both student writing ability and comprehension of the material covered in the assignments. As class enrollments increase, particularly at the introductory level, however, instructors often sacrifice writing assignments because of the difficulty in grading written assignments. In this study, we assess the benefits of an online writing program (Calibrated Peer Review [CPR]) that allows students to learn to evaluate and actually grade each other's essays. We find that the CPR process results in better content mastery on exams. Moreover, it makes writing assignments feasible in very large classes.