Captive propagation is the deliberate breeding of wild animals in captivity in order to increase their numbers. Reintroduction is the deliberate release of these species into their native habitat. The Mongolian wild horse, Pere David's deer, and the American bison would probably have become extinct without captive propagation. Nearly all cases of captive propagation and reintroduction involve threatened or endangered species. Zoos are increasingly involved in captive propagation, sometimes using new technologies. One of these, allows a relatively common species of antelope to act as a surrogate mother and give birth to a rare species.

Once suitable sites are selected, a reintroduction can take one of three forms. Reestablishment reintroductions take place in areas where the species once occurred but is now entirely absent. Recent examples include the red wolf, the black-footed ferret, and the peregrine falcon east of the Mississippi River...