Reintroduction is the deliberate release of animals from captivity into the wild. It usually involves species that are endangered or extinct in the wild. In some cases, reintroduction involves a species that had been wiped out in a particular ecosystem, but survived elsewhere in the wild.

Beginning in the 1980s, biologists have learned that many mammals and birds need to learn a lot in order to survive in the wild. Thus, reintroduction programs have to be planned carefully, ensuring that the animals have the necessary survival skills. It is also necessary to study the animals after the reintroduction. The goals of this study include learning whether the animals are surviving and breeding, and what effects the reintroduction is having on the ecosystem, and to learn how to improve the process.

Ongoing or successful reintroduction programs:

• California Condor in California (ongoing)
• European Beaver in several places in Europe (successful)
• Eurasian otter (Lutra lutra) in the Netherlands (ongoing)
• European Lynx in Switzerland (successful), and other parts of Europe (ongoing)
• Griffon Vulture in the Massif Central in France (successful)
• Lammergeier in the Alps (successful)
• Lesser White-fronted Goose in Sweden and Germany (ongoing)
• Musk ox in Alaska (successful)
• Northern Bald Ibis in Austria and Italy (ongoing)
• Père David's Deer in China (ongoing)
• Przewalski's Horse in Mongolia (ongoing)
• White-tailed Eagle in Scotland (successful)
• Wisent in Poland, Belarus (successful) and other parts of Europe (ongoing)
• Wolf in Wyoming (successful)