  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 galaxies as Massive Compact Halo Objects (MACHOs), the
remnants of a first generation of pregalactic or protogalactic stars. Various
candidates have been proposed for such remnants and we review the many types of
observations which can be used to detect or exclude them. Claims to have found
positive evidence for some of the candidates have generally turned out to be
spurious or questionable, so the status of the MACHO scenario remains
controversial. However, it would be premature to reject MACHOs altogether and
further observations are likely to resolve the issue soon.
