Genetic diversity within a population is a critical parameter for conservation biologists to assess. One approach for examining genetic diversity is through the determination of effective population sizes. Animal Conservation recently published a review of genetic approaches for estimating effective population sizes. We felt this review was incomplete both methodologically and conceptually. Here we extend that review to include coalescent-based approaches for estimating effective population sizes. We then discuss different kinds of effective population sizes, including inbreeding, variance and eigenvalue effective sizes.