Progressing gradually from babbling to meaningful sentences is something most babies do naturally. But why is that? What gives infants this remarkable capacity, and what in the world - or in the mind itself - inclines a child to speak? John Locke's answer constitutes a journey along the path of language development, a tour that takes in all the stops - neurological and perceptual, social and linguistic - that mark the way to intelligible speech. As Locke shows, the first steps are inseparably perceptual and social. Because infants have a deep biological need to interact emotionally with the people who love and care for them, they study the movements of faces and voices and begin to reproduce these behaviours themselves. Locke retraces these steps as they lead to the development of the vocal, neural, and cognitive capabilities essential to language. He also shows how infants participate in this process, eliciting and structuring the social and linguistic stimulation they need for learning speech. To distinguish what comes naturally from what must be taught, Locke also considers language in a larger biological context. He examines comparative data on non-human primates and songbirds and looks at special human populations, including deaf, blind, autistic, brain damaged, and tracheostomized infants. This biolinguistic approach aims to raise questions about the evolution of linguistic capacity in the species.