It now seems clear that the future of cities will largely be developed within existing contexts, and any sustainable development must ask about the future of what has already been built and is waiting to play a new role in the urban landscapes of our time. Almost intact original buildings, preserved for reasons of economy or respect, appear embedded in newer buildings, exposed or hidden under plaster, as fragments of architectural forms shaped by the succession of functions, involved and integrated in new uses. But whatever the reasons that lead a building to change its use, what happens in the transition from one state to another is a process that reshuffles the cards in that web of relationships that constitutes the very essence of all architecture and concerns its distributive, architectural and constructive character. In a particular way, reuse deals with what, for a particular culture and at a particular time, is now irreparably devalued, having lost all economic interest and having not (yet) acquired a historical-artistic one. But in what way does reuse intervene in the existing? How does it relate to it?