This paper summarizes what is known about preventable causes of occupational cancer, including single agents, complex mixtures, and broad occupational associations. Epidemiologic methods have been very successful in documenting cancer risks associated with single agents. Epidemiologic data are most conclusive when an exposure-response relationship can be demonstrated. Examples of agents for which epidemiologic studies provide evidence of an exposure-response relationship include benzene and (concurrent exposure to) ortho-toluidine and aniline. Vinyl chloride and bischloromethyl ether are examples of associations between single agents and rare histologic types of cancer. It is more difficult to conduct epidemiologic studies to identify cancer risks associated with complex mixtures. Studies of diesel exhaust and lung cancer and metal machining oils are cited as having employed advanced industrial hygiene and epidemiologic methods for studies of complex mixtures. Elevated cancer risks have also been identified in broad occupational groups, including painters and dry cleaners. Epidemiologic case-control studies are often used to detect such associations but are limited in their abilities to detect the causal agents. Major gaps exist in knowledge of occupational cancer risks among women workers and workers of color. Because epidemiologic research measures illness and mortality that have already occurred, a positive study can be interpreted to represent a failure in prevention. The challenge we face in the next decade is to identify interventions earlier in the causal pathway (toxicologic testing, biomarkers of exposure or precancerous changes, institution of engineering and good industrial hygiene practices to reduce occupational exposure levels) so that occupational cancer can be prevented.