Technical Abstract: Cryptosporidium, an apicomplexan protozoan, is reported to infect persons in 106 countries and more than 150 species of other mammals worldwide. Estimates of prevalence in humans vary greatly because reporting is not universally required, diagnostic methods vary greatly, and many persons have no access to medical care or do not seek it. At least 325 water-associated outbreaks of parasitic protozoan disease have been reported in which North American and European outbreaks account for 93% of all reports and approximately 66% are from North America. In 16 European countries 7,960 cases of cryptosporidiosis were reported in 2005. In the USA 3,505 cases were reported in 2003, 3,911 in 2004 and 8,269 in 2005. The greatest number of cases was reported for children 1–9 years of age and for adults 30–39 years of age with a seasonal peak coinciding with the summer recreational water season, reflecting increased use of rivers, lakes, swimming pools and water parks. Recreational waterborne outbreaks (n=68) primarily associated with swimming pools and spray features have affected 4592 persons in Australia, Canada, Japan, New Zealand, Spain, Sweden, England, Wales, and Scotland. Another 68 similar recreational water outbreaks involving 14,679 persons were reported in the USA. In Thailand, health risks have been associated with exposure to urban canal water for recreational purposes where Cryptosporidium and Giardia are estimated to cause ~ 47% of diarrhea. In 3 canals receiving municipal, agricultural, and industrial wastewater there was a major presence of Cryptosporidium hominis, indicative of human, not animal sources. Likewise, the warm weather recreational use of water throughout the world is a temporal effect primarily associated with an anthroponotic cycle in swimming pools and other treated water venues. Despite evidence of ubiquitous contamination of freshwater lakes and rivers with Cryptosporidium, only 12 outbreaks of cryptosporidiosis have been associated with recreational use of these waters in the USA and England. These include 8 lakes, 2 rivers/streams, and 2 hot springs. Numerous studies in the USA, Canada, Scotland, Ireland, Germany, Finland, Israel, Australia, Japan, Taiwan, and Hong Kong have reported the presence and concentration of Cryptosporidium oocysts in surface waters destined for drinking water. Of 325 water associated outbreaks of parasitic protozoan disease documented worldwide, 23.7% were caused by Cryptosporidium sp. that either passed through filtered or unfiltered drinking water systems, or contaminated water distribution systems in small and large community water systems. Unfortunately most of these studies did not use molecular methods to verify the species or genotypes and therefore neither a human nor animal source could be identified.