The Missile Defense transfer Radiometer (MDXR) is a cryogenic transfer radiometer for providing calibrations of collimated infrared beams in users' cryo-vacuum test facilities against the NIST primary optical power standards. The MDXR has both a filter radiometer and a cryogenic Fourier transform spectrometer (Cryo-FTS), both of which operate in vacuum at a temperature below 20 kelvin, and interfaces to customer chambers through an end flange. The Cryo-FTS provides continuous spectral coverage from 4 μm to 20 μm. An onboard absolute cryogenic radiometer (ACR) provides spectrally integrated power measurements down to a noise floor of approximately 3.5 pW. An onboard 7 cm collimator and cryogenic blackbody serve as a stability monitor and a spectral calibration source for the Cryo-FTS. The MDXR has been used multiple times at a variety of facilities since it was first operational in May 2010.