The definition of toxicology is “Science examining the interactions between xenobiotics and biological systems dealing with the health effects” or “Science obtaining the innoxiousness limits of the chemicals”.  On the other hand as there are lots of subtopics of toxicology such as descriptive toxicology, clinical toxicology, environmental toxicology, industrial toxicology, forensic toxicology, ecotoxicology and analytical toxicology it will be more useful to make a definition related with the areas of these subtopics. The truth is that, every chemical shows its toxic effects related to its dose so this fact makes the area of toxicology wider day by day involving drugs, cosmetics, food additives, pesticides, household products and industrial chemicals. The sources, physical, biological and chemical characteristics of the chemicals, the transformations of these in the organism, toxicity mechanisms, toxic doses, the innoxiousness limits, poisonings and the therapy, isolation from the biological environments, qualitative and quantitaive analyses, risk analyses and regulations are the basic area of interest of toxicology.