This overview introduces proceedings of the Fifth Water Chlorination Conference convened to review technical reports on all aspects of the use of chlorination, its by-products and environmental effects. Authors raise the following issues: (1) the need to better detect, analyze, and track carcinogenic and mutagenic by-products; (2) the need to be able to moderate if not minimize chlorination to rates that are realistic for high-use waters and to be able to calculate and analyze what realistic means; (3) the need to assess and to anticipate the potential to produce chlorine resistant pathogens; and (4) the need to examine disinfectant alternatives, especially for municipal and industrial treatment, to minimize or prevent formation of toxic by-products in the chlorination-reactors of treatment plants.