For about a century after Giardia was named by Lambl in 1859, the occurrence of the organism in humans was not widely considered significant in explaining the etiology of disease. The organism is now recognized as a cause of disease that ranges from mild to severe and debilitating gastroenteritis. Beginning in 1965, drinking water became increasingly implicated as an important route of transmission. An outbreak occurred in a Colorado ski resort in that year. Sewage contamination of the well water supply was thought to be the source of the organism. From 1965 through 1984, 90 waterborne outbreaks with 23,776 cases of giardiasis were reported in the United States. The report discusses the etiologic agent and examines drinking water outbreak occurrence and control.