This book is about the transition from infancy to language: about how the affective lives of infants and what they know of persons, objects, and events in the world come together in the effort to acquire language. The focus is on the period that begins in the last quarter of the first year and continues through the second year of life. This is the time ordinarily referred to as the single-word period in language development. It is when infants begin to learn words and begin to acquire the power of expression in language.

Infants on the threshold of language are already quite successful, and have been for some time, in expressing affect. The importance of affect expression for regulating and communicating their internal states and relatedness to other persons has been well documented in the last two decades of research in infant communication and social and emotional development. Parents readily respond to a young infant's cries, whimpers, smiles, and chortles and depend on these signals for their caregiving and socializing practices. Since Darwin, we have assumed that these capacities for emotional expression are biologically determined. They provide signals for protecting and extending the species and help regulate physiological arousal and discharge. The 9-month-old infant has yet to learn language but is adept at deploying these capacities for expression.