"HOLMES is aimed at directly measuring the electron neutrino mass using the electron capture (EC) decay of 163Ho.The measurement of the absolute neutrino mass represents a major breakthrough in particle physics and cosmology. Due to their abundance as big-bang relics, massive neutrinos strongly affect the large-scale structure and dynamics of the universe. In addition, the knowledge of the scale of neutrino masses, together with their hierarchy pattern, is invaluable to clarify the origin of fermion masses beyond the Higgs mechanism.The innovative approach of HOLMES consists in the calorimetric measurement of the energy released in the decay of 163Ho. In this way, all the atomic de-excitation energy is measured, except that carried away by the neutrino. A finite neutrino mass m causes a deformation of the energy spectrum which is truncated at Q-m, where Q is the EC transition energy. The sensitivity depends on Q - the lower the Q, the higher the sensitivity - and 163Ho is an ideal isotope with a Q around 2.5keV. The direct measurement exploits only energy and momentum conservation, and it is therefore completely model-independent. At the same time, the calorimetric measurement eliminates systematic uncertainties arising from the use of external beta sources, as in neutrino mass measurements with beta spectrometers, and minimizes the effect of the atomic de-excitation process uncertainties.HOLMES will deploy a large array of low temperature microcalorimeters with implanted 163Ho nuclei. The resulting mass sensitivity will be as low as 0.4eV. HOLMES will be an important step forward in the direct neutrino mass measurement with a calorimetric approach as an alternative to spectrometry. It will also establish the potential of this approach to extend the sensitivity down to 0.1eV.The detection techniques developed for HOLMES will have an impact in many frontier fields as astrophysics, material analysis, nuclear safety, archeometry, quantum communication."