Evolution is typically measured as a change in allele or genotype frequencies over one or more generations. Consequently, evolution is difficult to show experimentally in a semester-long lab course because most organisms have longer generation times than 15 weeks. In this article, the authors present an experiment to demonstrate and study evolution using bacteria as a model system. This experiment allows students to observe evolution in action and it illustrates how easily pathogenic bacteria can evolve into resistant forms that are much more difficult to treat. The major goals of this experiment are to observe evolution in real time, to understand mutation and natural selection and how they cause the bacteria to evolve, and to provide opportunities for the students to use evolutionary terminology. The authors also address the specific methods for setting up and performing the lab. In addition, they address the results that are expected, the meaning of the results, how they assess students' understanding of the lab, and how they meet the goals of the experiment. (Contains 6 figures.)