The Configuration of the Geomagnetic Field.- Particle Motions in the Earth’s Magnetosphere.- Sources, Losses, and Transport of Magnetospherically Trapped Particles.- Magnetospheric Substorms: A Model.- Auroras and Associated Phenomena.- Magnetospheric Plasma.- The Role of the Geomagnetic Tail in Substorms.- Instabilities in the Magnetosphere (Theoretical Treatment).- Gyroresonant Wave-Particle Interactions.- Diffusion and Acceleration in the Earth’s Radiation Belts.- Electric Fields in the Magnetosphere and the Origin of Trapped Radiation.