The joint NASA/European Space Agency Cassini mission to Saturn will carry the Cassini Plasma Spectrometer (CAPS). This instrument consists of three sensors, including the Ion Mass Spectrometer (IMS), a data processing unit, and an actuator that rotates CAPS in order to view a large fraction of the space environment. The IMS makes high sensitivity, mass resolved measurements of the various ion distribution functions. This sensor represents a significant step forward in ion mass spectrometry. The design employs a unique cylindrically symmetric linear electric field region in which some fraction of the initial ions “reflect.” Since the time-of-flight (TOF) is independent of these ions’ energies, extremely high mass resolution measurements can be made while maintaining the very high sensitivity achievable with TOF plasma spectrometers. Ions and neutrals which do not reflect in this region are simultaneously analyzed in a straight-through TOF section. These two complementary techniques provide hot plasma composition measurement with mass resolutions (m/∆m) of >50 and ~8 for geometric factors of ~10-3 and ~10-4 [cm2 sr eV/eV] per 11° x 22.5° pixel, respectively.