Water distribution systems in the United States routinely carry chlorine residuals < 0.05 mg/L and invariably carry residuals > 0.1 mg/L. This article outlines some of the benefits of using chlorine as a disinfectant residual and considers system characteristics (system age and vulnerability) that support this application. Benefits accruing to maintenance of a chlorine residual include protecting against modest degrees of intrusion of contaminants, serving as a sentinel for the presence of such contamination, and reducing the potential for growth of heterotrophic organisms. Includes 17 references, tables, figures.