The CrIS (Cross-track Infrared Sounder) instrument collects IR spectral radiance data to calculate calibrated atmospheric temperature, pressure, and moisture profiles for the NPOESS (National Polar-orbiting Operational Environmental Satellite System) program. CrIS features a Michelson Interferometer with three spectral bands (SWIR to LWIR), each with a 3 by 3 array of circular sensing apertures at the focal plane. The optical design includes a folded Gregorian telescope after the interferometer, a field stop to define the sensing aperture array, and collecting optics to place the interferogram energy onto the photovoltaic HgCd detectors. Many trade studies and analyses were performed to determine the design of the optical system, including telescope configuration, pupil locations, elimination of channelled spectra, polarization sensitivity, stray energy rejection, and compact packaging. This paper will describe the trade studies and analysis performed during the design of the CrIS instrument optics.