  The transition between the nearly smooth initial state of the Universe and
its clumpy state today occurred during the epoch when the first stars and
low-luminosity quasars formed. For Cold Dark Matter cosmologies, the radiation
produced by the first baryonic objects is expected to ionize the Universe at
z=10-20 and consequently suppress by 10% the amplitude of microwave
anisotropies on angular scales <10 degrees. Future microwave anisotropy
satellites will be able to detect this signature. The production and mixing of
metals by an early population of stars provides a natural explanation to the
metallicity, ~1% solar, found in the intergalactic medium at redshifts z<5. The
Next Generation Space Telescope (NGST) will be able to image directly the
``first light'' from these stars. With its nJy sensitivity, NGST is expected to
detect >10^3 star clusters per square arcminute at z>10. The brightest sources,
however, might be early quasars. The infrared flux from an Eddington
luminosity, 10^6 solar mass, black hole at z=10 is 10 nJy at 1 micron, easily
detectable with NGST. The time it takes a black hole with a radiative
efficiency of 10% to double its mass amounts to more than a tenth of the Hubble
time at z=10, and so a fair fraction of all systems which harbor a central
black hole at this redshift would appear active. The redshift of all sources
can be determined from the Lyman-limit break in their spectrum, which overlaps
with the NGST wavelength regime, 1-3.5 micron, for 10<z<35. Absorption spectra
of the first generation of star clusters or quasars would reveal the
reionization history of the Universe. The intergalactic medium might show a
significant opacity to infrared sources at z>10 due to dust produced by the
first supernovae.
