Publisher Summary In Greece, Drinking water comes exclusively from underground water sources. Mornos Reservoir, an artificial lake, is the largest source, with a water capacity of about 7.8 X 108 m3. This chapter presents a study in which a preliminary investigation about the occurrence of Giardia cysts and Cryptosporidium oocysts in Mornos Lake Reservoir as well as Aliakmon River has been conducted to estimate the potential risks due to protozoan parasites for populations supplied by large surface water resources. Current data indicate parasitic contamination of detectable level for the specific surface water supplies in Greece. Despite the detection of low numbers of Giardia cysts and Cryptosporidium oocysts in the Greek surface water supplies, the possibility of an outbreak at any time in the future always exists and the local water authorities should be in constant alert for coping with this danger.