With more than 1,400 species of bats described worldwide, the order Chiroptera is second only to rodents in ecological and taxonomic diversity. Bats play critically important roles in natural systems as seed and pollen dispersers, predators of invertebrates and vertebrates, and sanguinivores. The Central American countries of Costa Rica and Nicaragua have at least 123 species of bats (in nine families and 66 genera), or nearly 10% of the world's known species. Because of the importance of proper species identification for ecological and systematic studies and conservation efforts, we present a dichotomous key to the bats of this region. Our goal is the positive, in-hand identification of living bats that may be released unharmed after identification. Identifying Neotropical bats and understanding the taxonomic changes that affect the names used for the various species over time can be a challenge. This key includes the 123 species known to occur in Costa Rica and Nicaragua as well as three that are expected to occur in these countries but which have not yet been recorded. We provide illustrations of key characters useful for differentiating bats to species and updated taxonomic notes to assist the reader in assessing the literature.