An accurate and traceable photometric calibration is becoming increasingly important for large telescopes if they are to reach their full potential. The scientific community of the Gran Telescopio Canarias (GTC) has recognized this and there are programmes underway to ensure that suitable calibration standards are in place. The aim is to extend the methods used for the development of the photometric standards for the Infrared Satellite Observatory (ISO), which is described here, and develop full spectral energy distributions of the the calibration objects from which the magnitudes in any photometric system can be determined using synthetic photometry.