Comparative studies of vertebrate hearing organs have enabled an integrated approach to difficult questions related to function. Recent evidence for the independent evolution of similar hearing-organ specializations, in particular hair-cell differentiation, has helped identify common problems of hearing receptors and put them in a new perspective. Evidence that cochlear amplification is an ancient phenomenon has widened the search for the motor mechanism involved. In this regard, different hypotheses are best examined by making optimal use of natural structural variations. Studies on the evolution of the efferent system have provided new routes to investigate its function.