Agricultural land, the land that feeds the world, is in serious difficulty and it is not at all certain that it will be able to produce enough food for the world's population, in perpetuity. Confusion reigns about what sustainability is, but in this chapter we'll define some basic rules that will help people to understand it and to better manage their land. Does the green revolution deliver as promised? What is soil sustainability? How can we manage agricultural systems sustainably? How does the soil cycle work?