Fusion is the power source of the universe, since it is the process producing energy at the center of all stars, including our own sun. Inside stars temperatures above 10 million degrees allow for the hydrogen, the main constituent of stars, to fuse together to produce another element, helium. Stars can produce fusion energy for billions of years because hydrogen, its fuel, is by far the most abundant element in the universe, and because each fusion process releases staggering amounts of energy, about 10 million times the energy released in standard chemical reactions. It is precisely these features that make fusion so attractive as an energy source here on Earth. The fuels are so abundant in ordinary sea water as to make it effectively inexhaustible. The fusion process requires such high temperatures, and does not rely on a chain reaction, that it is inherently safe because intrusion from the outside world immediately eliminates the possibility of any further fusion. The fusion reaction itself produces no long-lived radioactive products – only helium. But like fission, it can produce large amounts of power in compact, central plants meaning that it can be deployed at a scale that would impact our energy portfolio in a decade timescale.