The Energetic Ion Composition Spectrometer (EICS) provides high sensitivity measurements of the mass and energy composition of the ambient energetic (0 eV to 17 keV/e) ion plasma. EICS consists of an electrostatic energy analyzer followed by magnetic sector designed to provide high sensitivity while resolving the principal ion species encountered in the magnetosphere. The sensor is mounted so that it views radially outward in the satellite spin plane, obtaining full pitch angle coverage twice each spin period. The angular resolution is +/- five degrees in the spin plane and somewhat broader in the direction perpendicular to the spin plane. The mass range from below the hydrogen peak to m/q>100 is covered in 64 mass channels. The energy range from spacecraft potential to 17 keV/e is covered in 64 energy settings, 32 logarithmically spaced energies with center energies from ~50 eV/e to ~17 keV/e, and 32 logarithmically spaced input retarding potential settings.