Dichloroacetate (DCA) and trichloroacetate (TCA) are important byproducts of drinking water chlorination. The combination of median concentration in drinking water and carcinogenic potency have elevated these two chemicals to pivotal positions for addressing the risks associated with the chlorination of drinking water. However, the concentrations at which these compounds induce tumors in rodents are orders of magnitude greater than those to which humans are exposed in drinking water. This paper reviews some of the most recent data that has been published on the metabolic and pharmacodynamic variables that can affect human and rodent sensitivity to the haloacetic acid group of disinfection byproducts in order to provide some perspective on how risks from these two byproducts might be treated in upcoming rules on disinfectants and their byproducts.