The public demand for the protection of human and environmental health has led to the establishment of toxicology as the science of the action of chemicals on biological systems. Toxicological research is focused presently very much on the elucidation of the cellular and molecular mechanisms of toxicity and the application of this knowledge in safety evaluation and risk assessment. This is essentially a predictive strategy (Fig. 1): Toxicologists study the action of chemicals in simplified biological systems (e.g., cell cultures, laboratory animals) and try to use these results to predict the potential impact on human or environmental health.