Language is a means of expressing knowledge and at the same time an object of knowledge. How do children, in addition to using Ianguage, acquire the capacity to think about language? The authors postulate that this ‘metalinguistic competence’ develops along with general cognitive structuration as described by Piaget. The present research concerns the concept of ‘the word’, experimentally studied with children aged 4–10 years. It shows a slow development of this concept from an initial indifferentiation between word and thing (at the level of metalinguistic thought and not at the level of know-how) towards the capacity of conceiving the word both as a meaningful constituent element of larger units and as a unit which is itself built up from smaller elements.