The nonadiabatic motion of geomagnetically trapped particles acrossLshells has been inferred from the measured distributions of trapped particles and has also been observed directly in a number of cases. This type of motion has important influences on both the steady‐state structure and the dynamics of the radiation belts. The characteristics of the radial motion have been evaluated and are generally consistent (within an order of magnitude) with the radial diffusion expected from fluctuations in the magnetospheric electric and/or magnetic fields. However, the complexity and variability of the observed phenomena suggest that several basic processes are acting simultaneously, and further efforts are needed to identify specific mechanism