There are two main types of bombs which release energy from the nuclei of atoms. The simplest kind is an atomic bomb. Like a nuclear power plant, it releases great quantities of energy through a process called nuclear fission, or 'splitting', of a large unstable (radioactive) element like uranium or plutonium. A more complicated type is the hydrogen bomb, or thermonuclear bomb, which releases an even greater quantity of energy through nuclear fusion, a process which has not yet been put to peaceful uses. The purpose of this page is to explain how each type of device works, from a physics perspective. The devices described here are the original designs, perhaps 50 years old, but the physical processes that cause the energy release remain the same in today's bombs and power plants.