Tolerance may be dened as the ability of organisms to cope with stress, either natural (such as temperature changes, salinity variations, oxygen level uctuations, and plant toxins) or anthropogenic, resulting from chemical input of many different classes of contaminants into the environment. Tolerance resulting from physiological acclimation acquired during the course of the life of an organism exposed to sublethal concentrations of contaminants is not inheritable. However, tolerance leading to a genetic adaptation in response to selection pressure in populations exposed to toxicants may be transmitted to the progeny. Resistance is frequently used in the scientic literature as a synonym for tolerance. Several authors have tried to clarify these terms, for example, Lotts and Stewart (1995) and Morgan et al. (2007), but the denitions they proposed were strongly different, and none of them is currently generally adopted. Nevertheless, the use of the term resistance is usually preferred by authors interested in the genetic basis of an organism’s ability to survive in a contaminated environment.