Most normal human cells undergo cellular senescence after accruing a fixed number of cell divisions, or are challenged by a variety of potentially oncogenic stimuli, in culture and most likely in vivo. Cellular senescence is characterized by an irreversible growth arrest and certain altered functions. Senescent cells in culture are identified by their inability to undergo DNA synthesis, a property also shared by quiescent cells. Several years ago, we described a biomarker associated with the senescent phenotype, a senescence associated β-galactosidase (SA-β-gal), which is detected by histochemical staining of cells using the artificial substrate X-gal. The presence of the SA-β-gal biomarker is independent of DNA synthesis and generally distinguishes senescent cells from quiescent cells. The method to detect SA-β-gal is a convenient, single cell-based assay, which can identify senescent cells even in heterogeneous cell populations and aging tissues, such as skin biopsies from older individuals. Because it is easy to detect, SA-β-gal is currently a widely used biomarker of senescence. Here we describe a method to detect SA-β-gal in detail, including some recent modifications.