  Quasars at large redshifts provide a powerful probe of structure formation in
the early universe. Several arguments suggest that the formation of ellipticals
and massive bulges may have involved an early quasar phase. At very large
redshifts, such structures are likely to be found at the highest peaks of the
density field, and would thus be highly biased tracers: the earliest (massive)
galaxy formation may have occurred in the cores of future rich clusters.
Preliminary results from our search for clustered protogalaxies around quasars
at z > 4 support this idea. Quasars at even larger redshifts may be an
important contributor to the reionisation of the universe, and signposts of the
earliest galaxy and cluster formation.
