Chlorine has been successfully used for the control of waterborne infectious disease for nearly a century. In the 1970s it was found that chlorine reacted with natural organic matter present in surface waters to produce disinfection by-products (DBP). Concern focused initially on the trihalomethanes (THM), but a wide variety of DBPs are now known to result from chlorination. Chlorination of drinking water has been one of the most effective public health measures ever undertaken. There are a number of alternatives to chlorination that are in active use in many parts of the world, but the risks associated with their by-products are even less well established than for chlorination. Moreover, the use of these alternatives vary in their effectiveness and some require greater sophistication in their application. This can mean less protection to public health as a result of inappropriate application and control. Therefore, hazards associated with the use of such a clearly beneficial process as chlorination must be carefully considered not only in an absolute sense, but also in the context of alternative approaches for producing a safe drinking water. The key question is whether the hazards associated with by-products have been sufficiently well established to warrant regulations that will undoubtedly have both positive and negative impacts on the public health. This symposium examined the toxicological and epidemiological data on chemical hazards associated with chlorination and attempted to measure this hazard against competing microbial risks. The first presentation discussed the available analytical epidemiological studies. A second presentation dealt with the importance of chlorination to the prevention of waterborne infectious disease. Pharmacokinetic, mechanistic, and modeling information on the prototypical DBP, chloroform, were discussed and contrasted with data on brominated THMs to determine if it was scientifically appropriate to regulate THMs as a single toxicological class. The fifth presentation dealt with the carcinogenic properties of a potent mutagen that is produced by chlorination. The final presentation discussed the haloacetates, carcinogenic DBPs whose concentrations approach and occasionally exceed those of the THMs. Clearly, there is a need to carefully weigh these different types and sometimes competing risks when considering the delivery of drinking water to ever-increasing populations for which there are finite sources of fresh water.