Fusion (more precisely nuclear fusion) is the process of joining together two smaller atoms to make a larger one, liberating energy in the process. This usually occurs between two helium atoms or one of its isotopes, such as deuteriumor tritium. This is what occurs in the sun and other stars. It is possible to fuse heavier elements, but it requires more energy, and only occurs in stars where the hydrogen is running out. Fusion formed all of the elements from hydrogen.

Attempts have been made to create a fusion reaction on earth, but the high temperature conditions mean a fusion reaction is very difficult to contain. Attempts have been made to use a magnetic field to contain the superheated plasma produced, but none have succeeded so far. Also research has gone into muon-catalysed cold fusion, a possible form of cold fusion that uses rare subatomic particles called muons to increase the chance of fusion and laser fusion,which bombards a pellet of fusion fuel with high-power lasers.The lasers both heat the fuel up and contain the plasma.