Publisher Summary This chapter provides the background for renewable energy sources and describes their occurrence, devices for using them, and the role of renewable energy in current and future energy systems, depending on social values and the economic rules used to judge the viability of competing solutions. It discusses the status of renewable energy development in the context of current political focus. Several economists have addressed whether it is cheaper to let the impacts of global warming happen and then try to reduce the consequences than to avoid the impacts. Arguments for the marginal cost advantage of adaptation as compared to abstract reference to mitigation appear even more suspect when one considers the energy efficiency measures not implemented. Several studies have shown that energy demands could be reduced by about a factor of four by efficiency measures fully available and costing less than the energy they displace, rendering exactly the same energy service to the user. The intermittency of several renewable energy flows makes it imperative to include features in systems relying on renewable energy inputs that can ensure supply–demand matching in time and place. This brings into focus bulk storage development, as well as means for transmission to and from such devices, and between load locations and supply locations, especially when the closest ones may be unavailable.