Cryptosporidium parvum is emerging as an important and problematic cause of diarrhea throughout the world. The lack of effective treatment or prophylaxis make the study of this organism a high priority. This review outlines the most recent developments in the understanding of cryptosporidiosis, especially with regard to pathogenesis, the immune response to infection, and the contribution of host factors to the symptoms of the disease. New paradigms of drinking water treatment and improved understanding of the immunopathogenesis of cryptosporidiosis are key to the control of this important emerging protozoal disease.