Many applications of animal cloning and genetic modification are controversial for environmental, health, animal welfare, and social reasons.

At least eighteen animal species have been cloned since 1996, when scientists produced Dolly the sheep, the world's first cloned mammal. Then and now, only a tiny percentage of cloning attempts produce live offspring. Many of these clones are unhealthy, and some leading scientists believe that none are "normal."

Nonetheless, animal cloning efforts continue. Some are justified as scientific experiments. Others are commercial ventures, either to produce pets for consumers or breeding animals for the livestock industry. The US Food and Drug Administration has approved the inclusion of meat and milk from cloned animals (without labels) in the  food supply.

Like cloning, genetic modification of animals is remarkably inefficient. It is being pursued for several purposes. Genetically modified (or transgenic) animals are commonly used in research. Efforts are underway to produce transgenic pigs as a source of organ transplants, transgenic fish for food, and transgenic livestock that resist animal diseases. In a practice sometimes called pharming, several mammalian species (cattle, sheep, and goats) have been genetically engineered to produce commercially useful human proteins in their milk. Fish engineered to glow in the dark have been developed and marketed as pets. Proposals to clone extinct species, particularly mammoths and neanderthals, regularly appear in the news media.