We begin with an overview of the experimental results for the temperature and
doping dependences of the optical-conductivity spectral weight in cuprate
superconductors across the whole phase diagram. Then we discuss recent attempts
to explain the observed behavior of the spectral weight using reduced and full
models with unconventional dx2−y2 charge-density waves.Comment: 17 pages, RevTeX4, 4 EPS figures; Invited paper for a special issue
of Low Temperature Physics dedicated to the 20th anniversary of HTS