Ninety waterborne outbreaks and 23,776 cases of giardiasis were reported in the United States during the period 1965-1984. These outbreaks occurred primarily in the northwestern, Rocky Mountain, and northeastern states, and most outbreaks occurred as the result of consuming contaminated surface water. In many water systems experiencing an outbreak, the current coliform standard was not exceeded, and the applicability of the coliform standard in preventing outbreaks of giardiasis must be reevaluated. Simple disinfection as the only treatment for surface water sources has been ineffective in preventing the waterborne transmission of giardiasis, and all surface water should receive effective filtration in addition to disinfection. Currently available methods for analysis of Giardia cysts in water samples depend upon microscopic examination of sample concentrates. All methods have four basic steps in common: concentration, purification, detection, and identification. Variations in procedures for accomplishing each of these steps have been reported but comparative evaluations under controlled conditions with a variety of waters have not been performed. Existing methods are not suitable for routine monitoring of water supplies in order to satisfy a standard.