The key target systems and organismal effects of ‘Fish Toxicology’ describe the key target organ systems for chemical impacts in fish, how chemicals produce cancer in these animals and how fish can develop resistance to chemical or drug toxicity. Therefore in fish, as in humans, livestock, companion animals, equines, wild animals and laboratory animals, pharmacology and toxicology cover all researches involved with basic and clinical pharmacology and toxicology. Human medicines may potentially induce similar pharmacological and toxicological actions in fish. Fish not only play an important role in the demand of food for humans but they are widely used as ‘model organisms’ for various biological experiments. Fish and other aquatic organisms are exposed from different substances like chemicals, drugs, toxicants or pollutants. Most of these substances or their metabolites cause pollution, leading to toxicity and various diseases including cancer. Therefore, the fishery scientists must develop the skill in recognizing the action and toxicity of such toxic substances in fish.