The human auditory system is sufficiently similar to those of other mammalian species, like small rodents, to allow one to take advantage of the knowledge about the main mechanisms of hearing obtained using invasive experimental techniques on laboratory animals. On the other hand, some of its extraordinary features, as the sharp cochlear tuning, which allowed humans to develop music and a complex language, are characteristic features of human hearing. Invasive diagnostic techniques are obviously unsuitable for human subjects. Therefore, non-invasive diagnostic techniques based on the measurement and analysis of otoacoustic emissions, although limited to the peripheral part of the auditory system, deserve special attention.