  CUPID is a proposed future tonne-scale bolometric neutrinoless double beta
decay ($0\nu\beta\beta$) experiment to probe the Majorana nature of neutrinos
and discover Lepton Number Violation in the so-called inverted hierarchy region
of the neutrino mass. CUPID will be built on experience, expertise and lessons
learned in CUORE, and will exploit the current CUORE infrastructure as much as
possible. In order to achieve its ambitious science goals, CUPID aims to
increase the source mass and dramatically reduce the backgrounds in the region
of interest. This requires isotopic enrichment, upgraded purification and
crystallization procedures, new detector technologies, a stricter material
selection, and possibly new shielding concepts with respect to the state of the
art deployed in CUORE. This document reviews and describes the rich and varied
R&D activities, performed inside and outside the CUORE collaboration towards
the next generation bolometric $0\nu\beta\beta$ experiment. A separate document
discusses the science goals and timescale for CUPID in more detail.
