PROBA-V is a remote sensing satellite mission for global monitoring of vegetation. It is designed to offer almost daily coverage of all land masses and to provide data continuity with the VGT2 instrument aboard SPOT-5. Accurate radiometric calibration is key to the success of the mission; therefore, a comprehensive system for in-flight radiometric calibration has been developed. Without no onboard calibration devices, this in-flight calibration will rely fully on vicarious methods. In total nine techniques for vicarious calibration have been implemented and tested in order to meet the radiometric mission requirements. Three key methods that contribute largely to the calibration performance are presented in this paper: Rayleigh, deep convective clouds, and cross-sensor calibration over stable desert sites. As the PROBA-V sensor has still to be launched, calibration algorithm verification is performed using data from the spectrally very similar SPOT-VGT1 and SPOT-VGT2 sensors.