Geospace features highly dynamic populations of charged particles with a wide range of energies from thermal to ultra-relativistic. Influenced by magnetic and electric fields in the terrestrial magnetosphere driven by solar wind forcing, changes in the numbers and energies of these particles lead to a variety of space weather phenomena, some of which are detrimental to space infrastructure. This book presents an overview of the latest discoveries and current scientific understanding of the coupling of electromagnetic waves and charged particles during magnetic storms, and explains the observed dynamics of these particle populations. The book furthermore includes investigations relevant to understanding and forecasting this space environment and the adverse impacts of space weather. High-energy electrons and ions in the Van Allen radiation belts and the ring current are of particular interest and importance with regard to the operation of space-based technological infrastructure upon which 21st century civilisation increasingly relies. This book presents the latest research on the sources, transport, acceleration and loss of these energetic particle populations, as well as their coupling during geospace magnetic storms.