This book brings together recent research on the ability of human listeners to discern changes in the shape of complex acoustic spectra. It systematically discusses issues surrounding the discrimination of a change in acoustic intensity and the physiological mechanisms responsible for this process. The book also proposes two new theories which attempt to explain spectral shape discrimination on the basis of more elementary auditory processes. The research described here is helping scientists gain a better understanding of auditory skills such as those involved in listening to music and speech.