A renewable energy source is one that is constantly replenishing itself, including power harnessed from the sun, wind, moving water, and geothermal sources. Energy use by human civilization is best understood in contrast to nonrenewable energy sources, including fossil and fissile fuels, which can be exhausted when resources are depleted at rates faster than they are reproduced. Renewable energy is human civilization’s oldest energy resource. For most of human history, civilization was powered by renewable energy, until the discovery of fossil fuels in the 18th century. Ecological economists differentiate renewable and nonrenewable energies using the concepts of stocks and flows. This permits a useful analogy for describing the sustainability of renewable energy. The consumption of stocks of fossil fuels, which can take hundreds of millions of years to accumulate, can eventually deplete energy reserves, much like spending savings in a savings account. The use of flows of renewable energy, which are constantly replenishing, is analogous to spending current income. This suggests that renewable energy sources are sustainable energy sources because they cannot be drawn down. The use of stocks of fossil fuels and uranium has implications for intergenerational equity, because the more of it we consume today means fewer energy sources will be available to future generations. The social and environmental issues of conventional, nonrenewable energy extraction and use are widely documented. In addition to concerns about future energy supplies, the burning of fossil fuels causes greenhouse gas emissions, the emission of criteria air pollutants, wastewater emissions, and water use for extraction, processing, and even cooling. Most research shows that renewables perform much better than conventional energy sources when comparing environmental metrics. Some renewables do pose impacts, such as burning biomass, which has high water use and pollution and poses air quality problems. Most renewables have increased land use requirements compared to nuclear and fossil fuels. The latter points to the critical need for geographers to engage in renewable energy issues, because increased demand for renewables translates to an increased need for land.