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agement strategies, such as within-family selection against recessive lethals or serious pathologies (Foose et al. 1986), would not be possible options. Here, we summarize current management techniques for maintaining genetic di- versity in captive populations and the genetic and demographic aspects of selecting captive individuals for reintroduction to the wild. We illustrate the use of these techniques with data from captive breeding and reintroduction programs for two avian species, the Guam rail (Rallus owstoni) and California condor (Gymnogyps califor- nianus), and two mammalian species, the black-footed ferret (Mustela nigripes) and golden lion tamarin (Leontopithecus rosalia). A few of the last rails were captured for a captive breeding program before the remaining rails, and most of the birds on Guam, were exterminated by the introduced brown tree snake (Boiga irregularis) (Witteman et al. 1990). The condor population was extremely small and rapidly declining when the last wild individuals were brought into captivity (Dennis et al. 1991, Wallace in press). A distemper epidemic reduced the only known wild ferret population to a few individuals that were used to begin the captive breeding program (Thorne and Belitsky 1989). The tamarin population was in danger of extinction due to the destruction of most of its Atlantic forest habitat in Brazil and illegal capture for pet trade (Kleiman et al. 1986). The rail project is a joint program of the U.S. Fish and Wildlife Service (USFWS) and the AAZPA's SSP; the condor program is directed by the USFWS with the advice of the Condor Recovery Team (Wallace in press); the ferret program is overseen by the Wyoming Game and Fish Department, the USFWS and the AAZPA's SSP; and the tamarin program is coordinated by the Golden Lion Tamarin Interna- tional Cooperative Research and Management Committee (Kleiman et al. 1986). What Do We Mean by Genetic Diversity? The genetic variation present in individuals, populations or species can be measured and compared in several ways (Hedrick et al. 1986, Lande and Barrowclough 1987). One common measure is the amount of heterozygosity. Most vertebrate individuals are diploid, that is, each has two alleles at every genetic locus. An individual inherits one of these alleles from its mother, via an egg, and the other from its father, via a sperm. Thus, a typical vertebrate individual is either homozygous (the two alleles are the same) or heterozygous (the two alleles are not the same) at each of its approximately 100,000 genetic loci (Gilpin and Wills 1991). The concept of het- erozygosity is illustrated in Table 1 with hypothetical data on the genotypes of 10 individuals at three genetic loci. At locus A, all 10 individuals are homozygous for the dominant allele A. At locus B, individuals 1, 3, 6, 7 and 9 are homozygous for the dominant allele B, individual 10 is homozygous for the recessive allele b, and individuals 2, 4, 5 and 8 are heterozygous with one B allele and one b allele. At locus C, only individual 2 is homozygous. The heterozygosity of an individual can be estimated as the average heterozygosity across the number of loci for which we have data (Hedrick et al. 1986). From our example, individual 4 has the highest heterozygosity (2 of 3 loci are heterozygous 0.67). The heterozygosity of a population (H bar) is the individual heterozygosities averaged over all the individuals within the population (Table 1: H bar = 0.43; Hedrick et al. 1986). Typically in mammals, population heterozygosity is about 4 percent (Nevo 1978). 264 ? Trans. 57" N. A. Wildl. & Nat. Res. Conf. (1992)