Learning to speak and understand language is a remarkable and important accomplishment of early childhood. Parent-infant interaction in the first three years is critical to this. This is the focus of this review - the first of the whole area. Seven databases were searched and 1750 studies reduced to 60 with good evidence. These 60 were from many disciplines and their methodological quality was variable. There was strong evidence that parental contingency (communication when the intended recipient is fully oriented towards receiving and processing it) and parental efforts in pre-literacy activities are important. There was strong evidence that interventions can be effective. Parental elaboration of language had less strong evidence. The role of gesture and the role of interaction with siblings and peers had the weakest evidence, but even these were quite well supported. Requirements for future research are specified. Parent-infant interaction is already established as a key factor in language development.