Epidemiological issues related to occupational carcinogens and mutagens were reviewed. A search was conducted in the Registry of Toxic Effects of Chemical Substances database; 654 substances were discovered which had reviews from the International Agency for Research on Cancer (IARC) and exposure estimates from either the National Occupational Exposure Survey or the National Occupational Hazard Survey. The IARC's rating system, in which carcinogens were classified according to evidence of carcinogenicity as sufficient, limited, insufficient, and lack of evidence, was discussed. Although epidemiological studies were considered useful to the study of occupational carcinogenicity, in-vitro mutagen assays were regarded as the more efficient and cost effective study type. Based on the modified Ames test assay, most animal carcinogens were found to be mutagenic. However, bacterial mutagenicity was not viewed as a consistent indicator of mammalian mutagenicity. Determining the mutagenicity of a substance was considered important not only because of mutagenic effects, but also because of the link between mutagenicity and carcinogenicity. Several mutagenicity tests were reviewed, such as tests which used human peripheral lymphocyte cells as the substrate. Other pertinent issues to mutagenicity tests, such as confounding, choice of the referent group, and utility in large scale screening programs, were also addressed. The author concludes that further study of mutagenicity and carcinogenicity is crucial, since many workers are exposed to thousands of unevaluated chemicals. More epidemiological studies are also needed to better classify the substances currently listed as possible and probable carcinogens.