Otoacoustic emission (OAE) testing enables us to identify cochlear hearing disorders and to monitor changes in cochlear status undetected by other audiometric tests. The outer hair cells receive descending input directly from contralateral and ipsilateral auditory brainstem structures through medial olivocochlear (MOC) efferent fibers. The efferent system has been known to play a role in detection of signals in noise and protection from noise-induced cochlear damage. The contralateral acoustic stimulation (CAS) can assess noninvasively the status of efferent system function. By comparing OAE levels in the presence and absence of CAS, the emitted responses provide the most direct means of observing a major efferent effect on OHC activity. The CAS method has been shown to induce a reduction in the levels of both TEOAEs and DPOAEs of about 0.5 to 1 dB in the overall emission level. Techniques for measuring MOC effects using OAEs seem to have promise in clinical applications. They may predict the susceptibility to acoustic injury based on the robustness of efferent activity, and can be used in screening for individuals most at risk in noisy environment or in monitoring of noise effects in hearing-conservation programs. A lack of efferent-related activity may be an early indication of cochlear damage from noise exposure. Contralateral suppression of OAEs can be used in tinnitus subjects for measuring the function of OHCs and cochlear efferents. The CAS efferent test is also sensitive to abnormalities in auditory brainstem pathways that involve the descending cochlear efferent system. The efferent test may become an important part of the assessment of more central auditory processing disorders. Further studies are needed in order to apply these interesting techniques with high standards for their better use in clinical field.