  Massive quiescent galaxies at high redshift can shed light on the processes
of galaxy mass assembly and quenching of the star formation at early epochs. We
present observer-frame color-color diagrams designed to identify candidate
quiescent galaxies from z=2.5 up to the highest redshifts, that can be then be
selected for spectroscopic follow-up observations. The application to the
COSMOS2015 catalog shows that, after refining the selection with SED fitting,
the number of massive old quiescent galaxies exceeds the forecast of
state-of-the-art semi-analytic models, pointing out the need of an improvement
of the implemented quenching mechanisms at high redshifts.
