An increasing amount of support is accumulating for the hypothesis that the outer hair cells (OHC) of a mammalian cochlea give rise to an enhanced sensitivity and markedly sharp tuning of the mechanical response of the cochlear partition. The enhancing and sharpening effects of the OHCs are postulated to arise from a bidirectional transduction mechanism whereby not only a mechanical signal applied to the hair bundle is (forward) transduced into electrophysiological signals, but also an electrophysiological signal applied to the hair cell is (reverse) transduced into generation of mechanical forces and related displacements. This paper will review experimental evidence for the hypothesis and attempt to integrate results of various experimental and theoretical studies into a coherent framework.