Telomere length influences numerous cellular processes such as senescence, carcinogenesis and aging. Quantitative FISH (Q-FISH) is a comprehensive method that to measure the length of individual chromosome telomeres in single cells with a resolution of 200 base pairs. The method is based on the use of a peptide nucleic acid (PNA) telomere oligonucleotide probe and suitable digital image processing software to detect and quantify fluorescence signals. The length of telomere is directly related to its integrated fluorescence intensity, as it is assumed that PNA probes hybridize quantitatively to telomere repeats. For the accuracy of Q-FISH measurement, it is important to use suitable internal controls such as fluorescence beads of defined size to avoid inaccuracies due to variations of lamp intensities. The fluorescence intensity of the beads is then used to correct the fluorescence intensities of the telomer signals. The telomere/ centromere ratio or calibration relative to cultured cells with known telomere length allows the conversion of fluorescence intensity values into units of DNA length (RTLU). However, this step is not strictly necessary, as fluorescence measurements in arbitrary units (TFU) provide accurate results provided that the internal control (i.e., fluorescent beads) is used correctly.