Measurements of the acoustic input impedance of the human eardrum, or tympanic membrane (TM), have been used as indicators of middle-ear sound transfer for over 70 years. Modern measurements include (i) middle-ear input impedance in single- and multi-frequency tympanometry, (ii) impedance-based estimates of ear-canal reflectance, a description of sound power absorption at the TM, and (iii) laser-Doppler measurements of the sound-induced velocity of specific points on the TM surface. The advantages and disadvantages of these techniques are discussed, and their use in identifying and estimating the effects of different middle-ear pathologies is illustrated. A new technique, opto-electronic holography, which allows measurements of the volume displacement of the visible surface of the TM, is also discussed. Common themes are (i) the impedance of the middle ear varies widely within populations of normal hearing subjects and (ii) input-impedance based estimates of sound transfer through the middle ear are hindered by the presence of flexibility within the ossicular chain. It is concluded that while middle-ear input impedance may adequately describe the acoustic load on hearing-aid receivers, this impedance, by itself, is not always an accurate indicator of pathological alterations in sound transfer through the human middle ear.