Publisher Summary This chapter describes the forces within the nucleus, the conditions under which a nucleus becomes unstable and radioactive, and the relationship of nuclear stability to the abundance of the elements. It discusses the types of nuclear changes, such as α, s, and γ decay, of natural radioactive elements. The chapter explains the bombardment reactions by which many radioactive isotopes and the synthetic elements can be synthesized. It illustrates the fission and fusion reaction. Nuclear fission and nuclear fusion bring nuclei into the area of greater stability by increasing the binding energy per nucleon. Nuclides above the stable nuclei curve undergo radioactive decay mainly by positron emission or electron capture. Nuclides below the stable nuclei curve undergo radioactive decay mainly by electron emission. The chapter describes the nuclear power plants in use at the present time and those that may become important in the future.