  Primordial black holes (PBHs) of stellar mass or heavier would accrete
baryonic gas, which becomes denser and hotter, injecting energetic photons in
the cosmological medium soon after cosmic recombination, in the so-called dark
ages. The ionisation history of the universe would be altered, an effect cosmic
microwave background (CMB) temperature and polarisation anisotropies is
sensitive to. The magnitude of the effect depends on the abundance and mass
distribution of the PBHs, as well as on still uncertain aspects of accretion
luminosity. We review the current understanding of this field, with an emphasis
on the peculiarities of the phenomenon in the cosmological context, and present
the existing constraints on the PBH abundances.
