Detection of senescent cells using a cytochemical assay was first described in 1995 (Dimri et al., 1995). The identification of senescent cells is based on an increased level of lysosomal β-galactosidase activity (Kurz et al., 2000). Cells under normal growth condition produce acid lysosomal β- galactosidase, which is localized in the lysosome. The enzymatic activity can be detected at the optimal pH 4.0, using the chromogenic substrate 5-bromo-4-chloro-3-indolyl β D-galactopyranoside (X-gal) (Miller, 1972). In comparison, upon senescence, the lysosomal mass is increased, leading to production of a higher level of β-galactosidase, termed senescence-associated β-galactosidase (SA-β-gal) (Kurz et al., 2000). The abundant senescence-associated enzyme is detectable over background despite the less favorable pH conditions (pH 6.0) (Dimri et al., 1995). The SA-β gal positive cells stain blue-green, which can be scored under bright-field microscopy. In this assay it is best to avoid over-confluency of the cells, or cells that have undergone too many passages, as these conditions can cause false positive results.