The human language is unique in that we can refer to objects, events and ideas. The combination of syllables and words enables humans to generate an infinite number of expressions. An important prerequisite for language is the ability to imitate sounds, i.e. to store acquired acoustic information and to use this for one's own vocal production. Cortical structures in the brain play a crucial role in this. While songbirds and certain marine mammals are capable of such vocal learning, there is very little evidence for vocal learning in terrestrial mammals -- not even in our closest living relatives, the chimpanzees. Nonhuman primate vocal production is largely restricted to an innate repertoire of sounds.