The inner radiation belt is composed of geomagnetically trapped protons, heavier nuclei, and electrons from multiple sources and with multiple loss mechanisms. Trapped ions result from cosmic ray albedo neutron decay, solar particle injection, secondary production in the upper atmosphere, and ionization of anomalous cosmic rays. They are lost by Coulomb drag, nuclear scattering, radial diffusion, and geomagnetic secular variation. A numerical simulation of these processes will be described along with the required empirical inputs and a comparison of the results to satellite data.