Cytologically visible damage in human chromosomes can be detected as structural chromosomal aberrations, numerical changes in genome, sister chromatid exchanges (SCE) or as micronucleated cells. The importance of in-vivo cytogenetic damage that is induced in human cells is that it indicates that similar alterations may have occurred in other tissues, either in somatic or in germinal cells. SCEs represent symmetrical exchanges between sister chromatids; generally, they do not result in alteration of the chromosome morphology or the genetic information. Although the detection method is highly sensitive as an in-vitro screening test, in monitoring studies, it seems to be restricted to cases where the exposing agents are strong alkylating compounds (e.g., ethylene oxide, cytostatic drugs) or to some multi-exposure conditions (e.g., cigarette smoking, laboratory work, rubber industries). Micronuclei arise from acentric chromosome fragments or lagging whole chromosomes. They have been detected in a variety of dividing human cells, including exfoliated epithelial cells, bone-marrow cells and lymphocytes. Positive responses of induction of cells with micronuclei have been obtained in studies with cells of the buccal mucosa (chewers of betel or tobacco) or cultured lymphocytes of some groups occupationally exposed to agents like styrene or ethylene oxide.