Assessment of the likelihood of risks to humans, livestock, and wildlife from potentially toxic constituents in sewage sludge applied to land requires a knowledge of the potential for transfer of each constiuent from the sludge or sludge-soil mixture to crops and to animals. Transfer of sludge constituents from soil to crops is predominantly a function of: (1) the constituent; (2) soil pH; (3) characteristics of the applied sludge and cumulative sludge application rate; and (4) the crop species and cultivar grown. The extent of increase in trace element concentration above control for crops grown on a sludge-amended soil is very strongly affected by crop species. Cd uptake by Graminae can have a lesser response to soil pH change than other species, depending on the availability of soil Fe. Several methods have been used in different nations and at different times to estimate the maximum cumulative Cd application which protects the health of individuals.