As sensory cells, hair cells within the mammalian inner ear convert sounds into receptor potentials when their projecting stereocilia are deflected. The organ of Corti of the cochlea contains two types of hair cell, inner and outer hair cells, which differ in function. It has been appreciated for over two decades that although inner hair cells act as the primary receptor cell for the auditory system, the outer hair cells can also act as motor cells. Outer hair cells respond to variation in potential, and change length at rates unequalled by other motile cells. The forces generated by outer hair cells are capable of altering the delicate mechanics of the cochlear partition, increasing hearing sensitivity and frequency selectivity. The discovery of such hair-cell motility has modified the view of the cochlea as a simple frequency analyser into one where it is an active non-linear filter that allows only the prominent features of acoustic signals to be transmitted to the acoustic nerve by the inner hair cells. In this view, such frequency selectivity arises through the suppression of adjacent frequencies, a mechanical effect equivalent to lateral inhibition in neural structures. These processes are explained by the interplay between the hydrodynamic interactions among different parts of the cochlear partition and the effective non-linear behaviour of the cell motor.