This paper will discuss performance data of various mammalian ears, i.e., their overall sensitivities, frequency ranges, points of maximal sensitivity, directional patterns, localization by the binaural system, etc., in the light of a number of physical factors, such as head size, length of the ear canal, area of the tympanic membrane, and others. In general, these physical factors were found to be quite critical in determining the performance of the ear. Some must clearly be the result of a compromise between conflicting requirements: (a) to collect an optimal amount of acoustic energy over a wide frequency range, (b) to bring sensitivity close to its optimal value, (c) to avoid creation of dead zones in the directional pattern of hearing, and (d) to facilitate auditory localization both in the horizontal planes and in the vertical one. Its performance makes the ear an excellent early warning device, superior in this respect to all other sense organs. [Supported by NIH grants.]