:This article is about the elemental isotope. For the record label Helium 3, see Muse or A&E Records.

Helium-3 (He-3) is a light, non-radioactive isotope of helium with two protons and one neutron, rare on Earth, sought for use in nuclear fusion research. The abundance of helium-3 is thought to be greater on the Moon (embedded in the upper layer of regolith by the solar wind over billions of years) and the solar system's gas giants (left over from the original solar nebula), though still low in quantity (28 ppm of lunar regolith is helium-4 and 0.01 ppm is helium-3).http://www.moonminer.com/Lunar_regolith.html, The estimation of helium-3 probable reserves in lunar regolith It is proposed to be used as a second-generation fusion power source.

The helion, the nucleus of a helium-3 atom, consists of two protons but only one neutron, in contrast to two neutrons in ordinary helium. Its existence was first proposed in 1934 by the Australian nuclear physicist Mark Oliphant while based at Cambridge University's Cavendish Laboratory, in an experiment in which fast deuterons were reacted with other deuteron targets (the first demonstration of nuclear fusion). Helium-3, as an isotope, was postulated to be radioactive, until helions from it were accidentally identified as a trace "contaminant" in a sample of natural helium (which is mostly helium-4) from a gas well, by Luis W. Alvarez and Robert Cornog in a cyclotron experiment at the Lawrence Berkeley National Laboratory, in 1939. Lawrence and His Laboratory: Episode: A Productive Error