The Cosmic-Ray Isotope Spectrometer is designed to cover the highest decade of the Advanced Composition Explorer's energy interval, from ∼50 to ∼500 MeV nucl-1, with isotopic resolution for elements from Z≃2 to Z≃30. The nuclei detected in this energy interval are predominantly cosmic rays originating in our Galaxy. This sample of galactic matter can be used to investigate the nucleosynthesis of the parent material, as well as fractionation, acceleration, and transport processes that these particles undergo in the Galaxy and in the interplanetary medium. Charge and mass identification with CRIS is based on multiple measurements of dE/dx and total energy in stacks of silicon detectors, and trajectory measurements in a scintillating optical fiber trajectory (SOFT) hodoscope. The instrument has a geometrical factor of ∼r250 cm2 sr for isotope measurements, and should accumulate ∼5×106 stopping heavy nuclei (Z>2) in two years of data collection under solar minimum conditions.