The terrestrial radiation belts surround the Earth as two doughnut shaped rings separated by a region with few energetic charged particles. Solar events such as coronal mass ejections and strong interplanetary (IP) shocks can accelerate protons which enter the magnetosphere via the open field lines over the polar regions and can get trapped. The outer radiation belt is much more dynamic and electron fluxes often wax and wane on many timescales ranging from minutes to years. The radiation belts respond to IP shocks, which often energize electrons. This response to IP shocks is in a very different manner from the two processes which typically have timescales of a few to several days for the energized fluxes to reach their maximum or peak values. It must be emphasized that models of radiation belts did not anticipate the formation and persistence of a morphological feature. Subsequent modeling has come up with explanations ranging from radial transport to slow flux decay.