Animal testing has been used in medical research for a long time. Although the ethics of animal testing is a controversial issue, it is necessary for medical research. Animal research has played an important role in most of the medical discoveries of the last century. Animal testing offers scientists the opportunity to advance their ideas from hypothesis to execution. Since 1902, almost every Nobel Prize winner related to physiology or medicine has relied on animal experiments for their research. These research projects were based on some animals such as rats, rabbits, guinea pigs, cows and mice. For example, Robert Koch, Nobel Prize winner in 1905, was the first to discover that bacteria can cause infectious diseases. He demonstrated that a parasitic microorganism was responsible for a disease by isolating the microorganism in infected tissue in a pure container. Animals can serve as models for studying human diseases because humans and animals share hundreds of diseases. The animal study can be applied to humans, and anesthetics, tetanus vaccine, penicillin, and insulin have been widely used in humans. Animals are the most effective objects for experimentation because they are easy to control and reproduce. However, using animals is more accurate than other methods, and animal testing cannot be replaced or simulated because it has uncertain factors. Sacrificing a small number of animals can improve medical research and makes animal testing worthy and valuable. Experimental animals are protected and most countries try to minimize the pain suffered by the animal. They have regulations and laws to ensure that animals are used in the right way. Overall, animal testing is good science.