The program of any individual's vital cycle encoded in the genetic information is executed according to a time schedule that is the object of study of Chronogenetics. MZ twins provide the best evidence of the existence of this hereditary biological time and they represent, therefore, the natural test of Chronogenetics. Twin research can thus serve medicine in developing its most modern perspective, individual preventive medicine. Moreover, through approaches such as the Twins-Living-Apart Test, twin research can help to identify the influences of the environment on the health of man.