Central to the understanding of a substance’s toxicity is a consideration of its Absorption, Distribution, Metabolism and Excretion. How efficiently is a substance absorbed by the human body? How is it broken down? Which tissues does it reach and is it likely to bioaccumulate? How does the body eliminate the substance? All this comes under the general heading of toxicokinetics (TK). Generating new TK data in the laboratory involves the use of animals, is time consuming and extremely costly (particularly if radiolabels are required). It is possible, however, to make expert predictions taking into account what is already known about the toxicity of a chemical, its structure and its physico-chemical properties.