In this chapter, we present a very special kind of acoustic emissions, coming from inside the cochlea and generated along the basilar membrane by the electro-motile (active) vibrations of outer hair cells of the organ of Corti. They are called OtoAcoustic Emissions (OAE) and are detected in the ear canal by means of microphones which are usually assembled as part of earphone-like probes. Since their discovery by Kemp [1], the study of otoacoustic emissions has become an hot topic both in basic and clinical research, due to OAE unique feature to inform directly about the normal and pathological functions of the cochlear receptors mechanisms, thus like the efficiency of the middle ear transmission chain.