Book excerpt: Two groups of animals, bats and odontocetes (toothed whales), have independently developed the ability to orient and detect prey by biosonar (echolocation). This active mechanism of orientation allows these animals to operate under low light conditions. Biosonar is a conceptual overview of what is known about biosonar in bats and odontocetes. Chapters are written by bat and odontocetes experts, resulting in collaborations that not only examine data on both animals, but also compare and contrast mechanisms. This book provides a unique insight that will help improve our understanding of biosonar in both animal groups.