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Clouds and the Earth's Radiant Energy System (CERES) Flight Model 5 (FM5) instrument is designed to continue the long-term monitoring of the Earth's radiation budget. CERES has three sensor units that measure radiation in the 0.3 - 5.0 μm, 0.3 - >;100 μm and 8 - 12 μm wavelength regions. Prelaunch characterization of the sensors to determine their radiometric gain and spectral response function was performed in September 2008. The longwave calibrations was achieved using a Narrow Field of view Blackbody (NFBB) that is tied to the International Scale of 1990 (ITS '90). The calibration in the shortwave region is performed using the Shortwave Reference Source (SWRS) along with the Transfer Active Cavity radiometer (TACR). The spectral responsivity of the sensors is measured with narrow band filters for region below 2.5 microns and with a Fourier Transform Spectrometer (FTS) system above the 2.5 micron region. The calibration with CERES on-board sources were also performed during pre-launch testing which will serve as a traceability standard to carry the ground determined radiometric gains to orbit. This paper covers the CERES calibration methodology used in the ground calibration testing of FM5 sensors performed in 2008.