Unstable radionuclides decay to achieve stability by several methods—spontaneous fission; α, β−, β+, decays; electron capture and isomeric transition decay. The decay properties such as dependence on the neutron-to-proton (N/Z) ratios are discussed. Neutrino and antineutrino emission in β decays are presented. Internal conversion of γ-rays is explained. Decay schemes of radionuclides are presented. Auger process and Auger electrons in isomeric transitions are described.