The Met Office has been using data from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) on board the Meteosat Second Generation (MSG) satellites for nowcasting, assimilation in numerical weather prediction (NWP) and for providing guidance to forecasters since September 2004. This paper describes the cloud mask derived from MSG imagery which is used by all downstream products. The Met Office SEVIRI mask is compared with that of the Nowcasting Satellite Applications Facility (SAFNWC), and with the Moderate Resolution Imaging Spectro-Radiometer (MODIS) cloud mask. These comparisons show good agreement between the cloud masks: the Met Office and SAFNWC masks are in accord with the MODIS mask for 90.65% and 89.69% of SEVIRI pixels tested, respectively. © Crown Copyright 2011. Published with the permission of the Controller of HMSO and the Queen's Printer for Scotland