In the 15th century, Theophrastus Bombastus Hohenheim announced that everything is toxic; it is just a matter of dose. This is the one thing in toxicology that almost everyone agrees with. It follows, therefore, that every agent, within reason, ought to have some form of control whether by a recommendation on intake limits or an enforceable regulatory exposure standard. The Environmental Protection Agency (EPA) began risk assessment in the cancer area requiring the initial assessment to indicate whether there was enough basis to launch a full-scale investigation of an agent as a carcinogen. The EPA advocated the use of several mathematical extrapolation models, although it was realized that the cornerstone of quantitative risk assessment would become the linear nonthreshold dose-response model. Risk characterization is the component of the risk assessment that produces both the population and the individual risk estimates. It is obtained by multiplying the dose by the probability of response as derived from a dose–response model.