  In recent years, significant progress has been made in building new galaxy
clusters samples, at low and high redshifts, from wide-area surveys,
particularly exploiting the Sunyaev--Zel'dovich (SZ) effect. A large effort is
underway to identify and characterize these new systems with optical/NIR and
X-ray facilities, thus opening new avenues to constraint cosmological models
using structure growth and geometrical tests. A census of galaxy clusters sets
constraints on reionization mechanisms and epochs, which need to be reconciled
with recent limits on the reionization optical depth from cosmic microwave
background (CMB) experiments. Future advances in SZ effect measurements will
include the possibility to (unambiguously) measure directly the kinematic SZ
effect, to build an even larger catalogue of galaxy clusters able to study the
high redshift universe, and to make (spatially-)resolved galaxy cluster maps
with even spectral capability to (spectrally-)resolve the relativistic
corrections of the SZ effect.
