The Improved Stratospheric and Mesospheric Sounder (ISAMS) is an infrared spectroradiometer that formed part of the science instrument payload of the Upper Atmosphere Research Satellite. An essential part of the success of ISAMS in orbit was a program of prelaunch calibration and characterization of many aspects of the instrument's performance. A brief description of ISAMS is followed by a detailed discussion of the calibration and characterization methodology, the facilities used in this program, and the results from the spectral and radiometric measurements. The results are discussed in terms of factors affecting the in-flight performance of ISAMS, particularly the spectral response of the measurement channels, the radiometric linearity, stray radiations and their dependence on the line of sight view, signal-to-noise ratios, and the sensitivity of the in-flight radiometric calibration to anticipated changes to the thermal environment within ISAMS. Some of the “lessons learned” are discussed with reference to the ISAMS design and the design of future instruments and test facilities.