The Cassini Plasma Spectrometer (CAPS) Ion Mass Spectrometer (IMS) uses a unique electric field configuration coupled with precise sub-nanosecond timing to produce a high mass-resolution plasma analyzer with both broad energy and mass ranges. The IMS maintains a spectrographic mass resolution (m/Δm) of >50 over a mass range of 1 to 40amu for singly charged positive ions in the energy range of 0.001 to 15keV. Ions having energies between 15 and 50keV are measured with a lower mass resolution of -7.5 as are ions below 15keV which are neutralized or negatively ionized by passage through thin carbon foils at the entrance to the mass analysis section. Both high and medium mass resolution measurements are made simultaneously on incoming ions of all masses. The total angular acceptance is 11° x 160° and area! acceptance is ~1cm2 per pixel. An overview of the design and results of the laboratory testing of tne IMS are presented here.