  The formation of the first stars and quasars marks the transition between the
smooth initial state and the clumpy current state of the Universe. In popular
CDM cosmologies, the first sources of light started to form at a redshift z=30
and ionized most of the hydrogen in the Universe by z=8. Current observations
are at the threshold of probing the reionization epoch. The study of
high-redshift sources is likely to attract major attention in observational and
theoretical cosmology over the next decade.
