Although the structure of an enzyme is often depicted as static, it is dynamic. Hence, a population of chemically identical enzymes has not one, but a distribution of structures at any moment in time. Does this have an effect on the activity of the enzyme? This article reviews experiments designed to test the hypothesis that this distribution of structures results in a distribution of enzyme activities. The experiments reviewed here use different enzymes, falvin adenine dinucleotide, β-galactosidase, alkaline phosphatase, exonuclease I, lactate dehydrogenase I, α-chymotrypsin, the 20S proteasome, and horseradish peroxidase. All experiments come to the same conclusion, when measured individually, apparently identical enzymes show a distribution in rates of activity.