We share this planet, our home, with millions of species. Justice and sustainability both demand that we do not use more resources than we need. Restraint in resource use and living within nature's limits are preconditions for social justice. The commons are where justice and sustainability converge, where ecology and equity meet. The survival of pastures and forests as community property, or of a common good like a stable ecosystem, is only possible with social organizations with checks and controls on the use of resources built into their principles. The breakdown of a community, with the associated erosion of concepts of joint ownership and responsibility, can trigger the degradation of common resources.