Book excerpt: The definition of intelligence has become the object of many controversies - particularly about its nature and the causes of its development - with essential social implications at stake. To get out of this deadlock, the authors of this book propose a social conception of intelligence and of its development: they consider intelligence as resulting from the inter-individual coordinations of actions and judgements. They experimentally study how groups of children elaborate new cognitive tools which their members, taken individually, did not possess at the start, and how these cognitive tools are subsequently used by the child alone.