The cochlea works as an acoustical amplifier and a frequency analyzer. Scientists have been trying to explain its functioning for a long time. Last century, von Helmholtz was the first one to suggest that inside the cochlea there would be individual resonant elements, spatially organized and specific for each frequency. Differently, von Békésy observed that the vibration of the cochlear partition is not a simple resonance of independent components, but the different regions of the cochlea seems to be mechanically associated. In the last two decades the comprehension of how sound is processed inside the cochlea has evolved substantially. Recent experiments demonstrate that to achieve the extraordinary sensitivity and frequency tuning of the mammalian cochlea, some kind of a mechanical process must exist to boost energy and amplify the vibrations of the basilar membrane. It is believed that such mechanical active process is responsible for the generation of the otoacoustic emissions that can be recorded in the external auditory canal, and that the in vitro electromotility of the outer hair cells is the agent of this amplification process.