Neutrinos have been a constant in defying our expectations with respect to their properties in the standard model of particle physics and major questions as for example their mass-scale and ordering are still open as of today. As neutral leptons they could be either Dirac particles or Majorana particles, where the particle coincides with its antiparticle. The latter opens up new lepton number violating processes, in particular a double beta decay process with the generation of two electrons without corresponding electron anti-neutrinos. The most sensitive limits on the half-life of this process are at the level of 10²⁵-10²⁶ years, with the most stringent limit for Te-130 coming from the Cryogenic Underground Observatory for Rare Events (CUORE) at LNGS, Italy. In this talk, I will review the scientific motivation and then present the experimental design of this ton-scale experiment employing 988 detectors in a 15 mK environment. I will discuss the particular challenges of the cryogenic setup, also dubbed the "coldest cubic meter in the known Universe" and present recent science results from multiple years of operation. In the second half of the talk, I will focus on a future upgrade of CUORE to CUPID presenting several of the intense R&D efforts taking place in the US and Europe with a particular highlight of the R&D in France.