A literature review was conducted to evaluate the mechanisms mediating the fate ofGiardia lambliacysts andCryptosporidiumsp. oocysts in surface waters, particularly in lakes and reservoirs. Emphasis was placed on quantification of source and sink terms as applied in mass balance models. The literature review results indicated that cysts and oocysts [referred to collectively as (oo) cysts] are commonly detected over a wide range of concentrations in a wide variety of aquatic systems. Humans and other animals are considered to be the sources of (oo)cysts introduced to aquatic systems. Most studies included some measure of (oo) cyst viability but not necessarily infectivity. Sedimentation was identified as an important loss mechanism for (oo) cysts in lakes and reservoirs. There were general indications that ambient irradiation or pH levels would have little effect on (oo) cyst viability or infectivity, while temperature, drying, and redox levels may have more varying effects. The (oo) cysts would be expected to remain viable for longer periods than for fecal bacteria in similar circumstances. Kinetic submodels (as associated coefficients) required to quantify these phenomena are generally unavailable.