During the last 30 years, our concept of cryptosporidiosis has changed from that of a rare, largely asymptomatic disease, to an important cause of diarrhea in animals and humans worldwide. Significant disease first appeared in cattle. Subsequently, the zoonotic danger of the organism was recognized in HIV-infected persons and young children. Cryptosporidium are now ubiquitous and disease has been described in over 79 host species. Cryptosporidiosis has become a major cause of calfhood diarrhea worldwide. In humans it accounts for up to 20% of all cases of childhood diarrhea in developing countries and is a potentially fatal complication of AIDS. Waterborne contamination is a growing concern as a source of widespread outbreaks of disease. Factors that have contributed to the emergence of cryptosporidiosis in animals include biological features of the organism, the lack of an effective treatment or preventative, increased environmental contamination, and trends in livestock production. In humans the zoonotic nature of infection and an increased at-risk population have contributed to disease. Genetic characterization of Cryptosporidium, improved detection methods, and a better understanding of the factors that predispose to disease are important contributions to understanding the epidemiology of cryptosporidiosis.