Telomeres play an important role in ensuring the integrity of the genome. Telomere shortening can lead to the loss of genetic information and trigger DNA damage responses. Cultured mammalian cells have served as critical model systems for studying the function of telomere binding proteins and telomerase. Tremendous heterogeneity can be observed both between species and within a single cell line population. Here, we describe the assay that analyzes the average length of telomeres in cultured cells (TRF analysis).