Ions in the trapping region of the earth's magnetosphere are subject to physical and chemical interactions which control their absolute and relative abundances. Charge exchange reactions act to establish a distribution of ionic states that is largely determined by the chemical properties of the individual species. Convection (“drift”) mechanisms and cross- L diffusion cause ions to be distributed over the entire trapping region with flux intensities determined by the nature and strength of the ion source, transport and loss mechanisms which in general are dependent on energy, mass and charge. Current theories describe ion transport through path tracing for individual particles or by radial diffusion for a population as a whole based on stochastic analysis; a comprehensive treatment of the combined convection and diffusion for trapped and non-trapped ions is yet to be developed. Even in studies where diffusion is the sole transport mechanism considered, only equatorially mirroring particles ( α 0 = π 2 ) have been theoretically treated. There are clearly both upper and lower bounds on the ion energy beyond which diffusion theory ceases to be valid: at high energies where the ion gyroradius becomes too large for the adiabatic approximations to be valid and at low energies where convective drift is a dominant process. In spite of the known shortcomings of the diffusion theory and associated modeling, intriguing theoretical predictions of the relative ionic composition of the radiation belts have been made and some of them are now confirmed by direct observation. Among them is the predicted importance of ions heavier than protons at ring current energies of tens of keV which follows from the charge exchange chemistry.