The physics of the solar-terrestrial environment is governed principally by the interaction of energetic charged particles with electric and magnetic fields in space. Near the Earth, most of these charged particles derive their energy ultimately from the Sun, directly or through the interaction of the solar wind with the Earth’s magnetosphere. These interactions are complex and non-linear. The magnetic and electric fields that determine the motion of the charged particles are affected in turn by the motion of these particles. Moreover, the processes that occur on the microscale may affect the behavior of the system on the macroscale. Fortunately, there are approximate methods to treat these systems. We do not have to worry about the motions of the individual particles in most situations.