XMM instrument calibration is performed as a mixture of so-called ``forward'' calibration (i.e., using simulations of the instruments to predict their response with SciSim) and the classical calibration using arrays, like e.g., flat fields, describing the measured instrument characteristics. The results of the calibration analyses by the SOC are contained in the calibration files delivered to the user in the Current Calibration Files (CCFs) that come with each science data delivery.