The article reviews the effective population size (N_e) to actual population size (N) ratio in wildlife, emphasizing the importance of this ratio in predicting inbreeding and genetic variation loss. The study analyzed 192 published estimates from 102 species, identifying five major variables affecting the ratio: population size fluctuations, variance in family size, form of N used (adults, breeders, or total size), taxonomic group, and unequal sex ratio. Comprehensive estimates, which included all relevant variables, averaged only 0.10-0.11, significantly lower than previously recognized. The findings suggest that wildlife populations have much smaller effective sizes than previously thought, with implications for conservation and management criteria such as the Mace-Lande criteria for endangerment and minimum viable population size estimates. The study also highlights the need for careful specification of the form of N when reporting estimates and the adjustment factors required to estimate N_e from census numbers.The article reviews the effective population size (N_e) to actual population size (N) ratio in wildlife, emphasizing the importance of this ratio in predicting inbreeding and genetic variation loss. The study analyzed 192 published estimates from 102 species, identifying five major variables affecting the ratio: population size fluctuations, variance in family size, form of N used (adults, breeders, or total size), taxonomic group, and unequal sex ratio. Comprehensive estimates, which included all relevant variables, averaged only 0.10-0.11, significantly lower than previously recognized. The findings suggest that wildlife populations have much smaller effective sizes than previously thought, with implications for conservation and management criteria such as the Mace-Lande criteria for endangerment and minimum viable population size estimates. The study also highlights the need for careful specification of the form of N when reporting estimates and the adjustment factors required to estimate N_e from census numbers.