The Universe is bursting with energy. It explodes out of stars as light, races along wires as electricity and thunders through the air as sound. It floats undetectable, driving the expansion of the Universe, as dark energy. Humans harness energy however they can, from burning coal to harvesting the renewable energy of the Sun or the wind, and even from decaying atoms in nuclear fission. Nuclear fusion is a new frontier of energy with the potential for immense amounts of clean, renewable energy.