Animal testing / animal experimentation is the use of non-human animals in scientific experimentation. It is estimated that 50 to 100 million vertebrate animals worldwide — from zebrafish to non-human primates — are used annually. Although much larger numbers of invertebrates are used and the use of flies and worms as model organisms is very important, experiments on invertebrates are largely unregulated and not included in statistics. Most animals are euthanized after being used in an experiment. Sources of laboratory animals vary between countries and species; while most animals are purpose-bred, others may be caught in the wild or supplied by dealers who obtain them from auctions and pounds.

The research is conducted inside universities, medical schools, pharmaceutical companies, farms, defense establishments, and commercial facilities that provide animal-testing services to industry."Introduction", Select Committee on Animals In Scientific Procedures Report, United Kingdom Parliament. It includes pure research such as genetics, developmental biology, behavioural studies, as well as applied research such as biomedical research, xenotransplantation, drug testing and toxicology tests, including cosmetics testing. Animals are also used for education, breeding, and defense research.

Supporters of the practice, such as the British Royal Society, argue that virtually every medical achievement in the 20th century relied on the use of animals in some way,The use of non-human animals in research: a guide for scientists The Royal Society, 2004, page 1 with the Institute for Laboratory Animal Research of the U.S. National Academy of Sciences arguing that even sophisticated computers are unable to model interactions between molecules, cells, tissues, organs, organisms, and the environment, making animal research necessary in some areas. Some non-governmental organizations like PETA and BUAV question the necessity of it. These opponents make a range of arguments: that it is cruel, poor scientific practice, cannot reliably predict effects in humans, poorly regulated, that the costs outweigh the benefits, or that animals have an intrinsic right not to be used for experimentation.

The practice of animal testing is regulated to various extents in different countries. In 1984 the WHO's Council for International Organizations of Medical Sciences (CIOMS) issued International Guiding Principles for Biomedical Research Involving Animals, and developed countries have developed regulatory frameworks that are more extensive. In the United States and in many other countries, review committees serve as gatekeepers for determining whether the use of animals proposed is warranted. They examine protocols to see if these can be improved by reducing or replacing animal use, and aim to minimize suffering. As of 2008, the trend in developed nations to offshore trials in biomedical research has also affected preclinical testing on animals, although to a lesser extent than clinical trials on humans.