The effective population size (N_e) provides information on how fast genetic variation is being lost, or relatedness is increasing, in a population of interest. This parameter is often considered to be related to population viability. Genetic approaches offer several avenues for estimating N_e; recent developments have helped relax assumptions of closed populations and stable population sizes that have hindered the estimation of N_e in many wildlife populations. The most promising areas of development are in the assessment of temporal changes in genetic composition for the estimation of contemporary N_e and the application of coalescent theory for estimation of historical N_e. Application of most estimators still requires making some questionable assumptions about wildlife populations. In general, these methods will be most beneficial when used in conjunction with current and historical demographic information.