The epithelial barriers that form the body’s interface with the environment, and the cellular barriers that separate internal organs and tissues, have to be permeable to a wide variety of chemicals to allow nutrient absorption and waste product elimination. The permeability of these physiological barriers, however, also allows toxicants to penetrate. Exposure to toxicants through ingestion, inhalation, and dermal contact may therefore result in harmful absorption. The degree of adverse effects following exposure to a toxicant is dependent on the rate and extent of absorption into the body, distribution to susceptible organs and tissues, and specific interactions with biological targets. Living organisms are adapted to toxicant exposure through the development of transport and elimination mechanisms that prevent the build-up of harmful chemical concentrations in organs and tissues.