Population census size (N _C) and effective population sizes (N _e) are two crucial parameters that influence population viability, wildlife management decisions, and conservation planning. Genetic estimators of both N _C and N _e are increasingly widely used because molecular markers are increasingly available, statistical methods are improving rapidly, and genetic estimators complement or improve upon traditional demographic estimators. We review the kinds and applications of estimators of both N _C and N _e, and the often undervalued and misunderstood ratio of effective-to-census size (N _e /N _C). We focus on recently improved and well evaluated methods that are most likely to facilitate conservation. Finally, we outline areas of future research to improve N _e and N …