The mammalian cochlea converts sound into electrical signals through mechanical vibrations of the basilar membrane (BM). Sound waves enter the cochlea via the stapes, causing BM vibrations that travel along its spiral structure. High-frequency sounds are localized near the base, while low-frequency sounds reach the apex. Each BM site has a characteristic frequency (CF) to which it responds most strongly. BM vibrations move hair cell stereocilia, opening transduction channels and generating receptor potentials that excite auditory nerve fibers. At the base, BM motion exhibits a compressive nonlinearity, with responses to low-level stimuli being sensitive and sharply tuned, while intense stimuli are less sensitive and poorly tuned. This nonlinearity, including two-tone suppression and intermodulation distortion, suggests the presence of a cochlear amplifier, likely involving outer hair cell forces. At the apex, nonlinearities are less prominent, implying a lesser role for the cochlear amplifier. BM vibrations' properties explain most frequency-specific responses of auditory nerve fibers. The cochlea's mechanical processes involve the basilar membrane, organ of Corti, and tectorial membrane. Otoacoustic emissions and mathematical models of cochlear mechanics are discussed, but selectively. The cochlea's structure includes three membranous tubes, with the organ of Corti containing inner and outer hair cells that transduce vibrations into electrical signals. The BM's length is positively correlated with body weight, and its vibrations are influenced by cochlear fluids and hair cell transduction currents. In vivo studies of BM mechanics at the cochlea's base show that responses to tones exhibit compressive nonlinearity, with sensitivity and frequency tuning varying with stimulus intensity. At the base, BM responses to tones show compressive growth, with sensitivity increasing as stimulus intensity decreases. BM sensitivity curves show that sensitivity grows systematically larger at lower stimulus levels, with curves superimposing only at frequencies far from CF. The cochlear gain, calculated relative to stapes motion, shows that BM vibrations at the base have much higher gains than middle ear ossicles. BM responses to tones also exhibit phase lags that increase with frequency, with phase plateaus indicating distinct vibration modes. At the cochlea's apex, BM responses differ quantitatively from those at the base. Responses to tones show less compressive nonlinearity and are less sensitive. BM responses at the apex exhibit phase lags that increase with frequency, with phase plateaus indicating distinct vibration modes. BM vibrations at the apex show DC components and harmonic distortion, with harmonic distortion being lower in chinchilla than in guinea pig. Responses to clicks at the apex show transient oscillations at frequencies corresponding to CF, with compressive growth throughout their duration. The cochlea's tonotopic map maps CF to longitudinal positions on the BM, with CF increasing from apex to base. The mapping follows a mathematical equation, with CF values determined by cochlea length and BM position. The cochlea's mechanical behavior differs between apicalThe mammalian cochlea converts sound into electrical signals through mechanical vibrations of the basilar membrane (BM). Sound waves enter the cochlea via the stapes, causing BM vibrations that travel along its spiral structure. High-frequency sounds are localized near the base, while low-frequency sounds reach the apex. Each BM site has a characteristic frequency (CF) to which it responds most strongly. BM vibrations move hair cell stereocilia, opening transduction channels and generating receptor potentials that excite auditory nerve fibers. At the base, BM motion exhibits a compressive nonlinearity, with responses to low-level stimuli being sensitive and sharply tuned, while intense stimuli are less sensitive and poorly tuned. This nonlinearity, including two-tone suppression and intermodulation distortion, suggests the presence of a cochlear amplifier, likely involving outer hair cell forces. At the apex, nonlinearities are less prominent, implying a lesser role for the cochlear amplifier. BM vibrations' properties explain most frequency-specific responses of auditory nerve fibers. The cochlea's mechanical processes involve the basilar membrane, organ of Corti, and tectorial membrane. Otoacoustic emissions and mathematical models of cochlear mechanics are discussed, but selectively. The cochlea's structure includes three membranous tubes, with the organ of Corti containing inner and outer hair cells that transduce vibrations into electrical signals. The BM's length is positively correlated with body weight, and its vibrations are influenced by cochlear fluids and hair cell transduction currents. In vivo studies of BM mechanics at the cochlea's base show that responses to tones exhibit compressive nonlinearity, with sensitivity and frequency tuning varying with stimulus intensity. At the base, BM responses to tones show compressive growth, with sensitivity increasing as stimulus intensity decreases. BM sensitivity curves show that sensitivity grows systematically larger at lower stimulus levels, with curves superimposing only at frequencies far from CF. The cochlear gain, calculated relative to stapes motion, shows that BM vibrations at the base have much higher gains than middle ear ossicles. BM responses to tones also exhibit phase lags that increase with frequency, with phase plateaus indicating distinct vibration modes. At the cochlea's apex, BM responses differ quantitatively from those at the base. Responses to tones show less compressive nonlinearity and are less sensitive. BM responses at the apex exhibit phase lags that increase with frequency, with phase plateaus indicating distinct vibration modes. BM vibrations at the apex show DC components and harmonic distortion, with harmonic distortion being lower in chinchilla than in guinea pig. Responses to clicks at the apex show transient oscillations at frequencies corresponding to CF, with compressive growth throughout their duration. The cochlea's tonotopic map maps CF to longitudinal positions on the BM, with CF increasing from apex to base. The mapping follows a mathematical equation, with CF values determined by cochlea length and BM position. The cochlea's mechanical behavior differs between apical