Energy usage makes modern life possible. Without it we would have no communications, transportation, food, health, and many other services and products that we rely on daily. Energy issues are moving to the forefront in the 21st century because of constraints imposed by increasing energy needs, climate change, energy security, and the apparent decline of fossil fuel resources. The main challenge is how to provide more energy for more people worldwide while at the same time reducing greenhouse gas and pollution emissions and providing secure and plentiful energy supplies. There is no silver bullet, so a variety of energy resources and technologies will be required.