Inbreeding depression, accumulation and loss of deleterious mutations, loss of genetic variation in small populations, genetic adaptation to captivity and its effect on reintroduction success, and outbreeding depression are reviewed. The impact of genetic factors in endangerment and extinction has been underestimated in some recent publications. Inbreeding depression in wildlife and in the field has been clearly established, while its impact has been greatly underestimated. The size of populations where genetic factors become important is higher than previously recognized, as Ne/N ratios average 0.11. Purging effects have been overestimated as a mechanism for eliminating deleterious alleles in small populations. The impact of loss of genetic variation in increasing the susceptibility of populations to environmental stochasticity and catastrophes has generally been ignored. Consequently, extinctions are often attributed to "nongenetic" factors when these may have interacted with genetic factors to cause extinction.