When most biologists want to understand how evolution unfolds, they look for clues in the fossil record or the natural world. Richard Lenski simply walks across his Michigan State University lab to his freezers. Stored there are bacteria from a 25-year experiment in bacterial evolution. He started with 12 identical populations of Escherichia coli and over time has documented how evolution is roughly reproducible but also endless, even in a stable environment. Fifteen years ago, he almost abandoned it for digital models of evolution, then reconsidered—and was vindicated when his bacteria took one of their most dramatic evolutionary leaps. Now he hopes to keep the experiment going for many, many years more.