Bats and toothed whales both emit ultrasonic pulses and listen for returning echoes in a process known as echolocation. However, their biosonars are the results of independent evolution under conditions of poor lighting in air and water that offer very different conditions for production, transmission, and reflection of ultrasound. In air the sound speed is low, the wavelengths are short, and the absorption is high, whereas the opposite is the case in water. These different physical conditions provide the basis for the function and operation of biosonars to inform motor patterns during echo-guided search, approach, and capture of small prey items. Despite such vastly different physical frameworks and very different evolutionary starting points, recent field data reveal a striking functional convergence in the way bats and toothed whales independently evolved the capability to sense actively with sound in air and water. This convergence is in part driven by the often opposing physical effects of body sizes, sound speeds, and absorption in air and water, but also by a deeply rooted way in which the mammalian auditory system process sound at high repetition rates.