It has now become abundantly clear that in recent years a multiplicity of chemical, physical, and other agents to which man is exposed are capable of damaging his chromosomes. The effects of these agents on the human somatic and possibly germ cells have now been well documented, although not without some controversy over the interpretation and implications of the data. While biological and clinical significance of chromosomal aberrations are not as yet well understood, recent epidemiological evidence, from mutagen-exposed human populations, and recent experiments on viral-induced transformation of chromosomally abnormal cells, suggest an association between chromosome aberrations and oncogenesis.