Several years after the discovery of the Earth’s radiation belts (in the midsixties) the model describing their formation due to geomagnetic field fluctuations and/or the electrostatic convection field was developed. This model permitted to explain many regularities of the experimentally recorded radiation belt parameters. By the present time vast experimental data on the structure, dynamics and composition of the radiation belts has been acquired. These data permit to make more detailed comparison of physical model predictions and experimental results. This report analyses the experimental data on the ion composition of the radiation belts and corresponding physical models of ion sources and transport in the trapped radiation zone.