In three experiments using crossover designs, we investigated the effects of Classroom Response Systems (CRS) when presenting multiple-choice questions in real classrooms. In Experiment 1, students either used CRS for bonus points or simply saw the questions. There were no differences on a unit exam. In Experiment 2, students were told prior to a unit that they would either use CRS for course credit or no credit. Students using CRS for credit performed better on pre-lecture questions and a unit exam. In Experiment 3, students used CRS to answer pre-lecture questions for course credit or no credit. Students using CRS for credit again performed better on a unit exam. CRS appear to enhance learning when they encourage student accountability and increase readiness for lectures.