Monitoring for occupational exposures to carcinogens can be among the most daunting tasks of the occupational health professional. Air sampling data do not provide reliable estimates of exposure because the skin is often a major route of entry. Biological monitoring markers are available on several levels for chemical carcinogens, however. These can be used to augment the occupational health program by providing estimates of internal and effective dose. Carcinogen biomarkers have proven to be important tools in research studies of exposure and genetic susceptibility. The results of two of these studies will be reviewed and placed in the context of an occupational monitoring program. We found that a battery of markers was very useful in a cross-sectional study. Individuals with high exposure and effect could be identified as were physiological factors which caused higher levels of effective dose markers.