[en] Sister chromatid exchanges (SCEs) are cytological manifestations of DNA double-strand breakage and rejoining at homologous sites between the two chromatids of a chromosome. The occurrence of SCEs was deduced from the transformation of small ring chromosomes to large ring chromosomes following cell division. Using tritiated thymidine as marker and microautoradiography for detection, others demonstrated the occurrence of SCEs from the silver grain pattern on the sister chromatids. This method was eventually replaced by cytochemical methods. One showed that if cells were grown in medium containing 5-bromo-deoxyuridine (BrdUrd) for two cycles, the sister chromatids can be distinguished by the differential quenching of the fluorescence of the fluorochrome Hoechst 33258. Reduced staining with Giemsa stain of the BrdUrd-incorporated chromatids was also found to be useful in differentiating sister chromatids. The authors review in this chapter only those studies which have implications on the origin of SCEs