The hair cells are the receptor cells of the inner ear. There is still controversy concerning the mechanism of their activation. Studies on the hair cells of the bullfrog sacculus have provided much information on the activity of hair cells. However, the mammalian cochlea has two different types of hair cells – the inner hair cells (IHCs) and the outer hair cells (OHCs) – and it is likely that their activation mechanisms are not identical. Mechanical manipulations of the cochlea and measurements of the passive and active displacements of the basilar membrane in the normal and postmortem cochleas provide evidence that the OHCs are activated directly by the fluid pressures induced in the cochlea by low-level sound, and not indirectly by a passive traveling wave. The activated OHCs produce active displacements (the cochlear amplifier) which excite the IHCs, probably by deflecting their stereocilia, followed by excitation of the auditory nerve fibers.