The experience of the new town of Le Vaudreuil has been lived as a failure: this ambitious project of building a new town of 100 000 inhabitants 100 kilometers away from Paris and 25 from Rouen was to be both a “test operation of modern town planning and architecture” and a “pilot town for the environment”, but it only gave rise to a small commune of 13 500 inhabitants named Val-de-Reuil in 1995. On the basis of the experience of the so-called “Atelier de Montrouge”, a group of architects deeply involved in the new town project for nearly twenty years (1959-1978), we try to correct this interpretation. This micro-history reveals the long gestation of architectural and urban ideas, their theoretical foundations and the privileged modalities of their realization.