In a world studded with forgotten leftovers and abandoned buildings, there is a need to consider rethinking the traditional approach to design and architecture by imagining, instead of great new works, myriad precise interventions that bring all those spaces back to life appear forgotten and without value. Working on dismissed buildings is about multiple stories, forgotten beauties, and human absence. It's about fighting over-production and over-consumption in a world in which it's totally normal to buy and throw away things even if they are still new. Society needs to use what it's already built because it's enough. Society needs to use what it's already built by thinking of it in a new, more flexible, sustainable, and ethical way. Cross-disciplinary approach, short-time interventions, and low-cost interventions are the clue solution to make those abandoned architectural leftovers live again. This chapter proposes ethically, flexibly, and sustainably considering the variety and complexity of these spaces as a starting point and value and an effective opportunity to study contemporary urban dynamics.