Epidemiologic assessment of the cancer risk associated with trihalomethanes (THMs) began in the 1970s, motivated in part by the observation that tumors were produced when experimental rodent species were exposed to high levels of chloroform. Since the original discovery of chloroform and other THMs in treated waters, hundreds of other halogenated chemical species have been identified. The presence of such a large number of chemicals in a mixture that varies geographically and temporally poses a major challenge for risk assessment. Possible synergistic and antagonistic effects among these compounds are also not well understood. In this setting, epidemiologic data can be valuable for evaluating whether exposure to a mixture of disinfection byproducts poses a health risk, and if so, the nature of the dose-response relationship. This paper provides a summary of epidemiologic studies of the incidence of cancer in populations exposed to chlorination byproducts.