  Primordial black holes (PBHs) are a fascinating candidate for being the dark
matter, albeit one which has been heavily constrained. This review presents an
in depth look at those observational constraints, particularly at their nuances
and uncertainties. Despite their varied origins, the standard PBH formation
path is assumed to be collapse of perturbations after inflation, which should
leave signals visible in the CMB at certain scales. Other constraints come from
microlensing surveys, which severely limit PBHs as dark matter in the solar to
satellite range, but there are diminishing results in regards to lower mass
ranges. Gravitational waves signals and PBH evaporation from Hawking radiation
also make for useful probes, but the former requires the next generation of
experiments before making constraints beyond the solar mass range, and the
later is severely limited above $10^{-16} \rm M_{\odot}$. Other dynamical and
accretion constraints exist for PBH of large masses. Care also has to be given,
as all these constraints can carry different implications coming from
differences between monochromatic and extended mass distributions, and their
degree of clustering. Beyond all these issues, a window still exists for
primordial black holes to be all of the dark matter between $10^{-16}$ and
$10^{-11} \rm M_{\odot}$
