Genetic considerations are important both to the initial success of reintroduction programmes and to their long-term viability. Genetic contributions to initial success will be maximized by choosing founders with low inbreeding coeffi cients and high genetic diversity that are well adapted to the reintroduction environment. Inbreeding and loss of genetic diversity are unavoidable in all closed populations and so reduce the long-term viability of reintroduction programmes. The severity of their impacts in isolated populations is inversely related to the effective population size (Ne) and increases with the number of generations. For example, the reintroduced population of lions, Panthera leo, in the Eastern Cape, South Africa, has a carrying capacity (K) of only 10-15 animals and will reach an inbreeding coeffi cient of at least 60% after fi ve generations (without immigrants), equivalent to fi ve generations of full-sib mating. Such a population will have reduced reproduction and survival rates and a substantially elevated extinction risk, owing to inbreeding. In the long term, most closed reintroduced populations of large terrestrial predators are expected to go extinct for genetic reasons, typically in combination with other threatening factors. To avoid such genetic deterioration, unrelated migrants need to be introduced at regular intervals. To avoid outbreeding depression, the immigrants must be from the same species and evolutionary signifi cant unit and have the same karyotype. If they are taken from within the former continuous range of the species and from a similar environment, the risks of outbreeding depression should be very low.