  Cosmological nucleosynthesis calculations imply that many of the baryons in
the Universe must be dark. We discuss the likelihood that some of these dark
baryons may reside in the discs or halos of galaxies. If they were in the form
of compact objects, they would then be natural MACHO candidates, in which case
they are likely to be the remnants of a first generation of pregalactic or
protogalactic Population III stars. Various candidates have been proposed for
such remnants - brown dwarfs, red dwarfs, white dwarfs, neutron stars or black
holes - and we review the many types of observations (including microlensing
searches) which can be used to constrain or exclude them.
