Microbes have been widely used in experimental evolutionary studies because they possess a variety of valuable traits that facilitate large-scale experimentation. Many replicated populations can be cultured in the laboratory simultaneously along with appropriate controls. Short generation times and large population sizes make microbes ideal experimental subjects, ensuring that many spontaneous mutations occur every generation and that adaptive variants spread rapidly through a population. A highly useful experimental feature is the ability to preserve and store ancestral and evolutionarily derived clones, which can be revived in parallel to allow measurement of the competitive fitness of a descendant in comparison to its ancestor. The extent of adaptation can thereby be measured quantitatively and compared statistically by direct competition between derived and ancestral forms. Thus, adaptation need not be a matter of qualitative speculation, but is a quantitatively measured variable in these systems. Replication allows the quantification of heterogeneity in responses to imposed selection and thereby statistical distinction between changes that are systematic responses to the selective regime and those that are specific to individual populations.