Cellular senescence is a complex process associated with irreversible cell cycle arrest. We can distinguish replicative senescence, which is telomere dependent and stress-induced premature senescence (SIPS), which is telomere independent. Replicative senescence can be observed in culture after a few weeks or months, depending on the cell type. On the other hand SIPS can be observed a few days after treating with a senescence inducing agent. Till now a universal marker of senescence has not been decribed. Studies concerning senescence are possible thanks to the existance of many markers of senescence which enable to observe molecular as well as biochemical changes associated with this process. The presence of a few markers of senescence allows us to be sure that cells underwent senescence.