Résumé : The LHC data-taking will resume in 2015 with energy of 13-14 TeV and luminosity of 2÷5×1034 cm-2 s-1. At those energies, a considerable fraction of the particles produced propagate in the high pseudo-rapidity regions. The proposal for the upgrade of the CMS muon forward system involves Gas Electron Multiplier (GEM) chambers to be installed during the second LHC Long Shutdown (LS2) covering the pseudorapidity range 1.5 <