Radiation-damage tracks in solids from energetic nuclear particles are caused by ionization that leads to atomic disordering along particle trajectories. The process that transforms missing electrons into atomic disorder is termed an ion explosion spike: The ionized region is unstable because the Coulomb repulsion of adjacent ions overcomes the local bonding. Natural tracks can be of many origins, but for samples on earth, spontaneous fission of U-238 ( the key to fission-track dating of minerals), is by far the dominant source. There are occasional, localized exceptions, three of which are noted. Exotic sources are listed, including those that can operate for materials that are outside the protective shielding of the earth's atmosphere. (C) 2004 Kluwer Academic Publishers.