A fundamental structure of sounds encountered in the natural environment is the harmonicity. Harmonicity is an essential component of music found in all cultures. It is also a unique feature of vocal communication sounds such as human speech and animal vocalizations. Harmonics are produced by a variety of acoustic generators and reflectors in natural environments, including vocal apparatuses of humans and animal species as well as music instruments of many types. Given the widespread existence of the harmonicity in many aspects of our hearing environment, it is natural to expect that it be reflected in the evolution and development of the auditory systems of both humans and animals, in particular the auditory cortex. Recent neuroimaging and neurophysiology experiments have identified regions of non-primary auditory cortex in humans and non-human primates that exhibit selective responses to harmonic pitches. Accumulating evidence has also shown that neurons in many regions of the mammalian auditory cortex exhibit characteristic responses to harmonically related frequencies beyond the range of pitch. Taken together, these findings suggest that a fundamental organizational principle of the primate auditory cortex is based on harmonicity. Such an organization can play an important role in speech and music processing by the brain. It may also form the basis of the preference for particular classes of music and voice sounds.