Diagrams and figures play a central role in science and science education. Research has indicated that, when presented and used properly in a classroom setting, these external representations can contribute to students’ understanding of scientific concepts; however, it is apparent that students do not always use, understand, interpret, or value external representations as their instructors intend. In this paper, the authors examine how general chemistry students use and interpret an external representation in the form of a common energy-level diagram. The results include implications for any college science instruction that uses external representations.