OAEs provide non-invasive, objective imaging of the peripheral hearing function, and, in conjunction with accurate theoretical modeling of the hearing system, allow one to test separately different parts of the system. Acoustic signals may be less sensitive to noise, artifacts, and interference with respect to electric signals measured, e.g., with surface electrodes, thus high SNR may often be achieved with relatively fast measurement sessions. The availability of accurate deterministic models of the generation of OAEs, based on differential equations, permits, in principle, to use high-SNR OAE data to get high-quality diagnostic information. The following list of diagnostic applications is by no means exhaustive, it represents a collection of examples of OAE-based diagnostics, meant to highlight, when possible, the added value provided by advanced acquisition and analysis techniques based on state-of-the-art theoretical knowledge. Generally speaking, the OAE level is correlated to the sensitivity of hearing, the OAE phase to transmission properties of the middle and inner ear, and to the frequency resolution of hearing. The diagnostic use of OAEs is not limited to audiology. For example, the tight connection between the hearing system and the neural system makes it possible to develop OAE-based diagnostic techniques dedicated to the study of neurodegenerative pathologies.