Clouds and the Earth's Radiant Energy System (CERES) instrument, with it's three scanning thermistor bolometers, was designed to provide accurate measurements for the long-term monitoring of Earth's radiation energy budget. The sensors measure broadband radiances in reflected shortwave and emitted outgoing longwave energy, the two components of Earth radiation budget. Flight Model 5, the sixth of the CERES instrument is scheduled to fly aboard the NPP spacecraft on October 2011. Four of the CERES instruments are currently operational aboard the NASA Earth Observing System (EOS) Terra and Aqua spacecraft, acquiring radiance measurements for the past 11 years. CERES instrument consists of three sensor units. The first one is a Shortwave (SW) sensor measuring reflected solar energy in the 0.3 to 5.0 micron region. The second sensor called the Total measures both reflected and emitted thermal radiances covering 0.3 to >100 microns and the third one is a narrow band thermal window sensor making measurements in the 8 to 12 micron wavelength region. The pre-launch accuracy goal for the CERES instrument measurements is to have the emitted longwave radiances within 0.5% and the shortwave radiances within 1.0%. A thorough characterization of the CERES sensors prior to launch along with an on-orbit calibration of the sensors with built-in calibration sources are used to achieve this goal. Accurate pre-launch characterization of the sensors is vital in providing longterm stable Earth radiation data records and meticulous determination of the radiometric gains and the spectral response functions of the sensors is key to achieve this goal. The CERES instruments were able to accomplish this by conducting rigorous sensor calibrations in the Radiometric Calibration Facility (RCF) at the Northrop Grumman Aerospace Systems in Redondo Beach, CA. The RCF was designed to calibrate all CERES instruments and include an accurate Narrow Field of View Blackbody (NFBB) with an out field of view mask, Shortwave Reference Source (SWRS) system with minimum longwave variations and good spectral characterization, a cold space reference source, a point response function source and a cryogenically cooled Transfer Active Cavity Radiometer (TACR). The Narrow Field of View Blackbody (NFBB) with an emissivity of 0.99995 serve as the primary source for CERES sensor calibration. The Platinum Resistance Thermometers (PRT) tied to the International Scale of 1990 (ITS '90) provide precise knowledge of the NFBB source. The longwave calibration of the total and the window sensor is performed using the NFBB source. NFBB is also used to calibrate the TACR which is the transfer standard of the NFBB radiometric scale to the SWRS system. The SWRS along with the TACR is used to determine the radiometric responsivity of the Shortwave sensor and the shortwave portion of the Total sensor. The spectral characterization of the CERES sensors were also conducted during the pre-launch testing. The spectral response estimates for the wavelength region below 2.5 micron was achieved using the SWRS and TACR along with 13 narrow band filters, in combination with the measurements of the optical components of CERES sensors and the TACR. The spectral responsivity in the longwave region beyond 2.5 micron is determined using a Fourier Transform Spectrometer (FTS) system. The calibration using the on-board sources that will be used for post-launch calibrations of CERES sources are also performed during the pre-launch vacuum testing. The on-board calibration system known as the Internal Calibration Module (ICM) consists of two blackbody sources to calibrate the Total and Window sensors and a tungsten lamp for the Shortwave sensor calibration. The ICM calibrations serve as a traceability standard to carry the ground determined sensor radiometric gains to orbit. Since the CERES FM5 instrument was built in late 1990s as part of the EOS program, the instrument was placed in storage and underwent vacuum testing few times. The final pre-launch vacuum calibration for FM5 instrument was performed in September 2008. This 33 day long calibration was the most extensive one done on all CERES instruments. During this period, CERES sensors were calibrated using the primary sources and the ICM at different temperature settings. The sensor responsivities were determined for the hot and cold simulated orbital conditions. Based on the knowledge of the post-launch commanding and performance of the CERES instruments aboard Terra and Aqua spacecraft, additional evaluations were incorporated to support the post-launch calibration process. Unlike the previous EOS missions where two CERES instruments were flying on each spacecraft that enabled several cross comparison validation studies, FM5 will be a single CERES instrument on NPP spacecraft. Thus the additional pre-launch tests have enabled a better understanding of FM5 sensors responsivity to account for any changes that can occur on-orbit. The calibration philosophy for the CERES instrument and the sensor performance during the ground calibration testing of FM5 instrument will be discussed in this paper. The determination of sensor radiometric gain responses using primary sources will be presented. The performance of the sensors using the ICM sources at the instrument and spacecraft level test campaigns will also be shown.