Abstract : Senescence is a continuous and complex procès which takes place over the whole lifetime. Its genetic and biological causes are now better known, particularly thanks to the study of telomers outcome and the oxydative stress. The replicative senescence theory seems to explain an adaptative procès that accompanies oldness. Some markers such as the length of telomers, the loss of antioxydant activity and the accumulation of materials resulting from the cellular metabolism seem to be involved in this phenomenon and will probably be measurable soon. These parameters should be part of the biological assessment of the old person’s clinical status. Such alterations have physiological and clinical impacts from which result the vulnerability or often frailty that must be carefrully assessed because they greatly influence the old person’s stress response and could explain the phenotypical and heterogenous differences observed in the old population. Frailty is not irreversible, but can be treated by classical therapies or be delayed or prevented by various interventions. Physical exercise and well adapted nutrition seem promising. All these markers of oldness influence the functional decline and must be integrated as much as possible in the therapeutic decisions, which sometimes lead to heavy and complex treatments.