The design and fabrication of the infrared spectral imaging radiometer (ISIR) is presented. The ISIR was designed in 1994 to provide calibrated images in four thermal wavelength bands without cryogenic cooling by utilizing the new, uncooled microbolometer detector technology. The complete system was fabricated at Space Instruments, Inc. (SI) in 1995 and 1996 and delivered to NASA Goddard Space Flight Center (GSFC) for flight on the space shuttle in 1997. Photographs of the flight hardware are shown. The ISIR operates in a pushbroom fashion and utilizes real time, digital time delay and integration (TDI) to improve the signal to noise ratio. From a nominal shuttle altitude of 140 nmi, the nadir pixel subtends 240 by 240 meters on the ground. The size of the radiometer is minimized by the elimination of mechanical scan mechanisms and a space radiator. The ISIR instrument utilizes a through-the- optics calibration system to periodically obtain a two-point calibration for each pixel in the detector array. A blackbody with both heating and cooling capability is used to obtain accurate calibration data for both terrestrial and cloudtop measurements. The timeline logic, TDI integration, mechanism control, calibration, and data formatting are performed in the onboard digital processor which utilizes two microprocessors and seven programmable logic devices. The output data is recorded on two, 8 mm tape recorders.