This chapter discusses the meaning of sustainability and the principal sustainable energy sources: wind, solar, geothermal, and ocean (tidal, wave, and thermal). Their contribution to meeting world energy needs, along with their relative reliability compared to fossil fuels, will also be covered. Two sources of sustainable energy have already been dealt with: biomass in Chapter 3 and hydropower in Chapter 8. Some feel that nuclear power also qualifies as a sustainable energy source if reprocessing of spent fuel and breeding fuel can extend the life of uranium reserves for thousands of years.