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The field of modern toxicology nowadays involves itself in three main general areas of activities (Eaton and Klaassen, 1996). The first area is descriptive toxicology, in which toxicity studies are conducted in mostly experimental animals to provide necessary information for safety evaluation and regulatory requirements. The health risks of concern extend to those posed to fish, birds, plants, or other non-human populations, hence the subfield of ecotoxicology. Mechanistic toxicology is another general area in which continuing efforts are made to elucidate the mechanisms by which physical, chemical, or biological agents exert their toxic effects on living organisms. Such elucidation requires some basic knowledge in physiology, pharmacology, biochemistry, cell or molecular biology, and the like. Regulatory toxicology is the third general area in which the toxicity and the exposure data are applied to decide if a toxic agent (drug, pesticide, water or air pollutants, etc.) poses a sufficiently low risk to be tolerated or marketed for the stated purpose and in accordance with the law and order. Unlike the mostly bioassay-based toxicity data, the human exposure data typically are derived from epidemiologic investigations or using epidemiologic methods. As with epidemiology and other fields, toxicology can also be subdivided into many specialized areas with a more limited focus. Some of these specialized areas include chemical carcinogenesis, food toxicology, genetic toxicology, forensic toxicology, occupational toxicology, neurotoxicology, developmental toxicology, environmental toxicology, clinical toxicology, etc.