One of the most powerful tools in the analysis of toxicological events is the description of the intensity of the exposure in terms of dose, dose rate, or concentration for a specified duration of exposure in relation to the frequency or intensity of the observed responses. Dose-response relationships have received a generic treatment in all general textbooks of toxicology, such as those by Loomis (1974), Doull et al. (1980), and Hapke (1975). The statistical risk of experiencing an effect from an exposure to a chemical can be attributed to the interaction of several important factors. As noted in Chapter 1, this complex relationship of dose to response was already recognized as early as the 16th century by Paracelsus. To restate his observations as they apply in contemporary toxicology—a sufficiently high dose of any compound will produce severe adverse effects in all exposed organisms. Conversely, a sufficiently low dose of any compound will produce no significant effects that can be experimentally determined, no matter how sophisticated or extensive the experiment.