Animal testing or animal experimentation (including conducting surgery on living organisms for the purpose of observing internal structures, known as vivisection) is the use of living animals in scientific experiments, including genetics, neuroscience, psychology, and, controversially, developing cosmetics and investigating the safety and efficacy of medical treatments.
 Sciencific opinion
Despite difficult ethical questions, the consensus of the scientific community is that animal research is an effective and necessary method of scientific inquiry. It is an important step in determining the safety and efficacy of medical treatments before they are tested on humans. Proponents claim animal testing saves many human lives. For example, animals were the first recipients of open heart surgeries as the procedure was perfected; they suffered a near 100% fatality rate. That rate decreased over the course of two more rounds of experiments until by the fourth group it was zero.
Another thing that makes animal testing more efficient than human testing is that the former are much more controllable - the scientists are able to know their genetic predispositions, their eating habits, and basically everything about them - along with the animals being genetically identical in some cases. A common criticism is that the animals suffer, though this is rarely the case. Suffering alters the animal's hormone levels, which can severely damage the research, so they are handled in the best way possible so that they are calm, comfortable, and healthy.
Notwithstanding consensus on the usefulness of animal testing, scientists endeavor to reduce the number animals use and, where possible, replace the use of living animals altogether. A growing industry in computer modeling is attempting to create digital systems that can be an effective alternative, but this research has many decades to go until it can take the place of conventional animal models.
 Drawbacks to testing
The main drawback to animal testing is whether conclusions drawn apply equally or to some degree to humans. Drug companies with vested interests may happily over-extrapolate animal studies, while PETA and other groups play this point to death, often severely underestimating the efficacy of animal testing and ignoring the examples where it works well. The reality is somewhere in the middle where animal testing is extremely useful, but the results should be treated with some caution. Animal experimentation never guarantees the safety of human medicines, but the models often give very big hints and are suitable as a starting point; if it doesn't kill a rat, dog or chimp after a massive over-dose, it is very likely that it is safe to give a pill to a human, for example.
Experimentation on higher mammals may be tightly regulated. In the United States, the "Guide for the Care and Use of Laboratory Animals," a protocol at least adopted in part by most academic institutions, recommends conditions for the humane keeping, feeding, and euthanasia of animals used in experiments. Some countries require a license to experiment on animals, including the United Kingdom, which requires the submission and approval of a protocol and justification that no other reasonable test can be performed. Licensure is then strictly supervised. Additionally, law may regulate the breeding of animals used in scientific experiments to ensure humane treatment. The image of scientists just coming along and torturing animals for shits and giggles is just NOT how it happens.
 Animal rights protests
While the vast majority of people support animal research, some in the animal rights movement actively protest any use of animals in research. Extreme acts of opposition range from sabotage of research facilities, physical attacks on individual researchers and their families, and even murder and arson. Marketing efforts by animals rights activists often appeal to emotion by showing pictures of supposed cruelty to animals in labs (the most shocking pictures or horribly out of date, however), focusing especially on "cute" ones most associated with being pets, such as dogs, cats, and rabbits, though largely ignoring the rats and mice most commonly studied. Activists also ignore that scientists make great efforts to keep research animals in humane conditions and to minimize suffering. Violent activism requires additional security measures for scientists and their animals, as well as researchers refusing to speak about what they do - even if it is relatively harmless to animals - and where they do it for fear of attacks.
It should be noted that if PETA and the ALF get their way and animal testing is banned, it will only be banned in the countries where they have strong influence, i.e., the US and Europe, where strict regulations prevent unnecessary suffering of animals. As animal testing is essential, if banned in one place, it would just move to another; specifically places without such regulations.