  Cosmological nucleosynthesis calculations imply that there should be both
non-baryonic and baryonic dark matter. Recent data suggest that some of the
non-baryonic dark matter must be "hot" (i.e. massive neutrinos) and there may
also be evidence for "cold" dark matter (i.e. WIMPs). If the baryonic dark
matter resides in galactic halos, it is likely to be in the form of compact
objects (i.e. MACHOs) and these would probably be the remnants of a first
generation of pregalactic or protogalactic Population III stars. Many
candidates have been proposed - brown dwarfs, red dwarfs, white dwarfs or black
holes - and at various times each of these has been in vogue. We review the
many types of observations which can be used to constrain or exclude both
baryonic and non-baryonic dark matter candidates.
