A water utility's first priority is to provide their customers with drinking water that is free of disease-causing organisms. In situations in which pathogens are or may be present in tap water, the state or local public health agency may find it necessary to issue a boil-water advisory. The advisory typically recommends that customers boil all tap water intended for digestion. A decision to issue an advisory must usually be made within a short period of time and with either incomplete or inconclusive information. Nevertheless, such a decision must be made only after careful consideration. The Working Group on Waterborne Cryptosporidiosis (WGWC) was organized to promote the regular exchange of information about waterborne cryptosporidiosis and has developed guidelines for issuing and rescinding boil-water advisories. This paper discusses the boil-water advisory guidelines issued by the WGWC.