One of the objectives of gerontological research is to achieve, reproducibly and at will, a verifiable discrepancy between the chronological and biological age of organisms. To accomplish this, the experimenter must be in a position to measure biological age independently. In theory, this can be done in three ways: by actuarial analysis of large populations, assessment of overall morbidity, or observation of chronic degenerative changes that can be actually measured or graded according to a scale. Of these three approaches, only the last appears to be promising in experimental research. However, not all progressive degenerative changes represent practically useful parameters of biological age. Criteria for their evaluation are presented, and their theoretical prerequisites as well as concrete applications are discussed. In a more general way, one has to be aware that biological age is a statistical entity. It cannot be directly observed but only inferred from quantifiable epiphenomena, and is, as such, not measurable like temperature or weight.