The MeteoSat Second Generation (MSG) programme consists of a series of 3 geostationary satellites. The objectives for this programme have been defined by the European meterological community led by The European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) and the European Space Agency (ESA). The first MSG satellite (MSG-1) has been developed by ESA and is scheduled for launch in the year 2002. MSG-2 and MSG-3 will be procured by ESA on behalf of EUMETSAT. The MSG satellites are built by European Industries under ESA contract with Alcatel Space Industries being the prime contractor. EUMETSAT will procure the launch and operate the satellites. The MSG satellites will provide a continuous and reliable collection of environmental data in support of weather forecasting and related meteorological services. The purpose of this paper is to outline the In-Flight calibration of the main payload on-board the MSG satellites which is the imaging radiometer: Spinning Enhanced Visible and Infra-Red Imager (SEVIRI). After a brief description of the satellite and SEVIRI, this paper will focus in discussing the instrument radiance response and its Calibration using both the on-board calibration source and the in-flight foreseen vicarious calibration.