Cellular senescence, whereby cells cease to proliferate, is known to contribute to the aging process and age-related pathologies. It is elicited either by cell-intrinsic mechanisms such as progressive telomere shortening or due to the extrinsic stress-related factors, which via p53-p21 and p16-pRB tumor suppressor pathways signal cells to cease proliferation. A proper identification and characterization of senescent cells is necessary to understand the process of aging, age-related pathologies, and the development of therapeutics to treat age-related dysfunctions. The landmark discovery of Senescence-Associated-Beta-Galactosidase (SA-β-Gal) marker, and a simple colorimetric method to detect SA-β-Gal greatly facilitated identification of the senescent cells in human and rodent cells pertaining to age-related diseases (Dimri et al., 1995). Despite the availability of additional senescence biomarkers, the SA-β-Gal marker and histochemical detection method remain the most widely used tool to identify senescent cells in vitro and in vivo. Here, we revisit the original colorimetric method to detect senescent cells that was first published in 1995 (Dimri et al., 1995).