Publisher Summary This chapter discusses the space environment and interference. The region of space dominated by the plasma surrounding the earth is called the magnetosphere. The plasma in the magnetosphere can be divided into three regions: (1) the plasmasphere, containing cool plasma consisting of electrons, ions, and oxygen ions; (2) the plasma sheet, containing warm (5-keV) plasma, and (3) the radiation belts, with plasma energies up to several MeV. The auroral oval is that region where the plasma from the plasma sheet and radiation belts extends to low altitudes. The space environments for which spacecraft charging is a concern are encountered in geosynchronous and low-altitude, polar orbits. The spatial and temporal variations of the plasma environment in either the geosynchronous or polar regime are quite complex, and a large body of work has been devoted to characterization of those variations.