Well-designed "controls" distinguish experimental from non-experimental studies. Surprisingly, we found that a high percentage of students had difficulty identifying control experiments even after completing three university-level laboratory courses. To address this issue, we designed and ran a revised cell biology lab course in which students participated in weekly "experimental control exercises." To measure student understanding of control experiments, we developed a set of assessment questions; these were given to students prior to and following completion of either a standard cell biology lab course or the revised cell biology lab course. Not unexpectedly, the results indicate that the revised course led to greater improvements in students' ability to identify and explain the purpose of control experiments. Based on these observations, we recommend that explicit and detailed discussions designed to identify the design and purpose behind control experiments become a standard component of all laboratory courses.