In this paper we present the methodological approach developed by the Remote Sensing Unit (RSU) of Portuguese Geographic Institute (IGP) for the regular production and updating of land cover maps derived from medium spatial resolution satellite images in Portugal. The focus of this paper is to explain the adopted land cover nomenclature, the satellite images selection process, the classifier selection, the training and testing samples collection processes and final map accuracy assessment design. To tackle these issues, we produced a land cover map of Continental Portugal from multitemporal MERIS Full Resolution satellite images of 2005 and evaluated its accuracy. Overall accuracy of final map was of 80% with an absolute precision of 2% at the 95% confidence level. High independent accuracy assessment results demonstrate that medium spatial resolution satellite images can be used on an operational basis for annual production of land cover maps at the national scale.