Spontaneous sound signals emitted from the inner ear were first recorded by Kumpf and Hoke in 1970. Kemp reported phenomena of sounds which were emitted by the ear responding to acoustic stimulation. These "otoacoustic emissions" are supposed to be generated in the outer hair cells. Active contractions of the actin and myosin in these cells produce a frequency specific cochlear amplifier mechanism. Although the clinical value of spontaneous otoacoustic emissions is yet unclear, as well as role in tinnitus, the recording of click-evoked otoacoustic emissions has now become diagnostic routine. Click-evoked otoacoustic emissions cannot be recorded in ears with cochlear mid-frequency hearing losses > or = 25 dB. However, the use of sinus tones or distortion products as stimuli promises more frequency-specific results. Most important for the quality of measurement is complete closure of the external ear canal, correct positioning of the recording probe, maximal suppression of background noise and sufficient compliance of the patient, especially when testing children. Middle ear effusions also prevent recording. Responses to a standard of 260 stimulations are averaged and identified as "true" emissions by their sufficient reproducibility and characteristic pattern in frequency analysis. Currently, the recording of click-evoked otoacoustic emissions can be used to detect early discrete lesions of the outer hair cells. Their use as a screening tool concerning cochlear hearing disorders is already possible in newborn children. Isolated central hearing disorders still cannot be detected by this diagnostic procedure.