The molecular evolution is considered in several protein families. It can be studied with the data of an entirely known structure, like in hemoglobin or cytochrome, or of only partial structural data, as for proteases or immunoglobulins. In the case of isozymes the study of physico-chemical and kinetic properties is more indirect. Finally, it is shown that the molecular mechanisms are always the same, happening probably at the same frequency, the differences being due to a variable selection by molecular constraints and adaptation to the environment.