Nowadays, the parasitic zoonose organisms Giardia lamblia und Cryptosporidium spp. are among the most relevant pathogens of drinking water-associated disease outbreaks. These pathogens are transmitted via a fecal-oral route; in both cases the dose of infection is low. Apart from person-to-person or animal-to-person transmissions, the consumption of contaminated food and water are further modes of transmission. The disease is mainly characterized by gastrointestinal symptoms. In industrialized countries, the prevalence rate of giardiasis is 2-5 % and of cryptosporidiosis 1-3%. Throughout the world, a large number of giardiasis and cryptosporidiosis outbreaks associated with drinking water were published; in 2001 the first case in Germany was identified. Giardia and Cryptosporidium are detected in surface water and sporadically in unprotected groundwater. Use of these waters for drinking water abstraction makes high demands on the technology of the treatment process: because of the disinfectant resistance of the parasites, safe elimination methods are needed, which even at high contamination levels of source water guarantee safe drinking water. Further measures for prevention and control are implementation of the HACCP concept, which includes the whole chain of procedures of drinking water supply from catchment via treatment to tap and a quality management system.