Nuclear fusion is the process in which the nuclei of light elements combine, or fuse together, to give heavier nuclei. An example of a fusion reaction is that of two deuterium nuclei fusing together to give a helium nucleus. Deuterium is an isotope of hydrogen. The reaction is as follows: Deuterium +Deuterium--> Helium+energy Fusion reactions are accompanied by a much greater mass to energy conversion than in fission reactions. Nuclear fusion requires extremely high temperatures. This is because the small nuclei require enough kinetic energy to overcome their electrostatic repulsion. The energy we receive from the sun is from nuclear fusion. The sun is made up mainly of hydrogen and helium. Within the sun the temperature is millions of degrees Celsius, there is the constant fusion of small nuclei into larger nuclei.

In nuclear physics, nuclear fusion is a nuclear reaction in which two or more atomic nuclei collide at a very high speed and join to form a new type of atomic nucleus. During this process, matter is not conserved because some of the matter of the fusing nuclei is converted to photons (energy). Fusion is the process that powers active or "main sequence" stars.

The fusion of two nuclei with lower masses than iron (which, along with nickel, has the largest binding energy per nucleon) generally releases energy, while the fusion of nuclei heavier than iron absorbs energy. The opposite is true for the reverse process, nuclear fission. This means that fusion generally occurs for lighter elements only, and likewise, that fission normally occurs only for heavier elements. There are extreme astrophysical events that can lead to short periods of fusion with heavier nuclei. This is the process that gives rise to nucleosynthesis, the creation of the heavy elements during events such assupernova.

Following the discovery of quantum tunneling by Friedrich Hund, in 1929 Robert Atkinson and Fritz Houtermans used the measured masses of light elements to predict that large amounts of energy could be released by fusing small nuclei. Building upon the nuclear transmutation experiments by Ernest Rutherford, carried out several years earlier, the laboratory fusion ofhydrogen isotopes was first accomplished by Mark Oliphant in 1932. During the remainder of that decade the steps of the main cycle of nuclear fusion in stars were worked out by Hans Bethe. Research into fusion for military purposes began in the early 1940s as part of the Manhattan Project. Fusion was accomplished in 1951 with the Greenhouse Item nuclear test. Nuclear fusion on a large scale in an explosion was first carried out on November 1, 1952, in the Ivy Mike hydrogen bomb test.

Research into developing controlled thermonuclear fusion for civil purposes also began in earnest in the 1950s, and it continues to this day. The present article is about the theory of fusion.