Energy is a major driver in shaping the quality of the environment. On the one hand, energy creates environmental problems in its extraction, transportation, combustion, storage, and final consumption. Environmental problems are caused by activities as diverse as strip mining, oil spills, and nuclear-waste disposal. Environmentally more benign modes of coal, oil, and gas extraction and exploration are under development. The most environmentally troublesome aspect of the operation of the present energy system is its use of fossil fuels, which are today available at attractive market prices. Fossil-fuel use is at the core of many environmental problems, including urban air pollution, acid rain, ecological impacts of resource extraction, and global warming. The important directions for long-term energy R&D will be those which will "defossilize" the energy system.