When speaking to young children, adults adapt their language to that of the child. In this article, we suggest that this child-directed speech (CDS) is the result of a transactional process of dynamic adaptation between the child and the adult. The study compares developmental trajectories of three children to those of the CDS of their caregivers. Furthermore, a mathematical-conceptual model is built that captures the essential dynamics of adaptation in a series of coupled equations. This model is sensitive to changes in the language development of the child. The results show evidence for a dynamic form of adaptation, although there are also clear individual differences.