The progression from newborn to sophisticated language user in just a few short years is often described as wonderful and miraculous. What are the biological, cognitive, and social underpinnings of this miracle? What major language development milestones occur in infancy? What methodologies do researchers employ in studying this progression? Why do some become adept at multiple languages while others face a lifelong struggle with just one? What accounts for declines in language proficiency, and how might such declines be moderated? Despite an abundance of textbooks, specialized monographs, and a couple of academic handbooks, there is no encyclopedic reference work in this area--until now.