On Earth many nuclei are unstable, and they can reach a more stable configuration through radioactive decays. These nuclei are indicated as radioactive due to their emission of nuclear radiations. Depending on the emitted particles they are classified as alpha, beta, and gamma decays. Radioactive nuclei have been part of the Earth’s composition since the Earth’s formation or they are continuously generated by the interaction of cosmic rays with the atmosphere. The emitted particles lose their kinetic energy by interacting with Earth materials strongly contributing in this way to the geothermal heat. This capability of spontaneous energy production can also be efficiently used for applications in which the radioactive nuclei supply a nuclear battery, generating directly or indirectly an electric current. In this chapter the characteristics of radioactive decays are presented, the evidence of thermal heat produced by their decay in the Earth is discussed and some description of their use as a power supply for nuclear batteries is analyzed.