Thermonuclear fusion involves combining the nuclei of two or more light atoms to produce the nucleus of a heavier atom. Fusion requires tremendous heat and pressure to overcome nuclear forces, but liberates more energy than was used to initiate the reaction. Hydrogen bombs and stars demonstrate the power of fusion.

Several different fusion reactions exist, generally involving the fusion of various isotopes of one or both of the lightest elements, hydrogen and helium. Solar fusion involves a series of reactions that combine hydrogen nuclei (protons) to form helium nuclei, emitted neutrinos, positrons, and electromagnetic radiation (including heat and light) in the process. This type of 'proton-proton' fusion, although very efficient, is nearly impossible to achieve anywhere but inside a star, which creates the necessary pressures and temperatures in its core by virtue of its immense size. Human technology uses other means to produce a fusion reaction. In a hydrogen bomb, these conditions are achieved by exploding a nuclear fission bomb as a trigger, but the reaction is over in an instant. A self-sustaining fusion reaction that can power a city or drive a spacecraft is trickier.

The most successful method has proven to be magnetic confinement. Gas is ionized, forming a plasma, which is then squeezed by magnetic fields until it is hot and dense enough for fusion to take place. Fusion research initially concentrated on the deuterium-tritium (D-T) reaction, which required the lowest ignition temperature. This fuses two isotopes of hydrogen into helium, liberating vast quantities of energy in the process. The majority of its fuel is an isotope of hydrogen called deuterium, which is ordinary hydrogen plus an extra neutron. Deuterium is fairly common: in the form of deuterium oxide (heavy water) it forms one part in 5,000 of ordinary water, and can be distilled at some expense using electrolysis. Tritium is a rare radioactive isotope of hydrogen, but can be 'bred' by surrounding the fusion reactor core with a jacket of lithium, which transforms into tritium under neutron bombardment.

However, D-T fusion has a disadvantage: much of the energy liberated is in the form of energetic neutrons. Neutrons are dangerous and cannot be directly converted into electrical power. The neutrons must heat water, which produces steam, which drives a turbine, all of which adds extra bulk and cost. Moreover, the bombardment of neutrons irradiates and degrades the structural material of the reactor itself. Even with a careful choice of structural materials, this means a high maintenance and upkeep cost. Finally, tritium is an essential component in hydrogen bombs, and as such the global use of commercial reactors that require or breed tritium does not help nuclear nonproliferation. As a result, D-T fusion reactors failed to displace other types of power plants on Earth. A few were built as experimental systems, and some are still used in space, especially by the Red Duncanites, but in general, they have been superseded by D-He-3 reactors.

Second-generation fusion reactors fuse deuterium with helium-3, a rare isotope of helium. The He-3 reaction requires higher temperatures to ignite (and thus awaited the development of more advanced magnetic confinement technology), but its main products are charged particles instead of neutrons.

A D-He-3 reactor is environmentally safer, and does not require the same heavy shielding. (There is a tiny amount of radiation produced by secondary reactions, so some shielding is needed.) The charged particles are also easier to convert into electricity. This means a D-He-3 reactor can be lighter, more efficient, and more easily maintained.

The smallest present-day D-He-3 reactors mass several tons and generate megawatts of energy. Building-sized reactors generating a gigawatt of energy are common for cities, producing power that costs a few pennies per kilowatt-hour. D-He-3 reactors are also used in many spacecraft, space habitats, and colonies, powering energy-intensive processes such as agriculture, desalination, heavy industry, electrolysis, and terraforming. Fusion torch drives are variations on these reactors; pulse drives use different technology.