Structural changes in chromosomes were one of the first recognized forms of genetic injury resulting from exposure to physical and chemical agents. Depending on the lesions induced in the DNA, and therefore, on the nature of the genotoxic substances, damage to chromosomes falls into two categories, structural aberrations and sister chromatid exchange (SCE). Because these two end points respond differently to chromatin lesions, they are complementary in any study designed to identify potential exposure. The circulating lymphocyte in the human is an appropriate cell type in which to measure cytogenetic changes because it is readily accessible, carries genotoxic substances and their metabolites throughout the body, is long-lived, and can integrate exposure. In animal and human studies, it has been shown that both SCEs and aberrations can persist in the lymphocyte following acute or chronic exposure.