Low temperature micro-calorimeters at 0.1K have been proposed in 1985 as new tools for the full absorption and precise beta spectroscopy for finite neutrino mass searches by the group of Genova. At that time the micro-calorimeter technology was at the very beginnings and had spectral performance even worse than the one of the well established instruments for beta spectroscopy. In those years the conjecture of a  neutrino mass of tens of eV/c2 matching the existing cosmology models, pushed the first pioneering efforts in developing such kind of detectors. Presently, after the discovery of the flavor mixing involving finite mass states, the goal of such kind of measurements continues to appear very challenging. But this is mitigated by the giant step forward in the low temperature micro-calorimeter’s performance made by several groups over the world in the last decade.

Array of superconducting Transition Edge Sensor (TES) micro calorimeters as large as 1 K-pixels with 1-2 eV energy resolution in a few keV energy band are recently produced together with the dedicated multiplexed electronics. Meanwhile, isotopes to be used as suitable probe for the calorimetric neutrino mass experiment were studied: mostly Re-187 and Ho-163. HOLMES will set the first sub-eV sensitivity calorimetric measurement with the most updated Low Temperature Detector technology by means of high precision spectroscopy of the Electron Capture Decay of  Ho-163.

The TES detector technology we have developed for this project has been also applied to the Low Energy Nuclear Spectroscopy and the Fine Structure of Beta Decay (BEFS) that are undetectable with other detectors, Bolometers for Cosmic Microwave Background Measurements and focal plane detectors for X-ray astrophysics within ASI and ESA research programs.