When present at a suf¤ciently high concentration, a pollutant can cause adverse effects on an organism. To elicit damage to an exposed organism, the pollutant must ¤rst enter the host and reach its target site. A complex pathway exists between the time of initial toxicant exposure and the manifestation of damage induced in the organism. This chapter discusses general ways in which environmental pollutants exert their actions on plants, animals, and humans.