Why is sharing important to our civilization, our cities and the earth? It is critical, even essential to our survival, because without it, we will overconsume the planet’s resources and overheat it. It is of paramount importance that we find ways to increase prosperity without economic growth, or better yet, to achieve degrowth. Sharing our assets, our services and places, even our activities and experiences may be our best hope to reduce the human ecological, energy and carbon footprints. Whether a reformed version of capitalism or a fundamentally new economy, the sharing city and no-growth ethic form a large, profound and open question. In any case, from an energy use and emissions point of view, doubling energy and technological efficiency or doubling the intensity of asset use though sharing have much the same impact - whether it is cars, transit, homes, equipment, offices or workshops. This realization opens up a huge new opportunity for reforming or replacing neo-liberal capitalism with longer-term thinking and more humane economics.