  Primordial black holes (PBHs) are a profound signature of primordial
cosmological structures and provide a theoretical tool to study nontrivial
physics of the early Universe. The mechanisms of PBH formation are discussed
and observational constraints on the PBH spectrum, or effects of PBH
evaporation, are shown to restrict a wide range of particle physics models,
predicting an enhancement of the ultraviolet part of the spectrum of density
perturbations, early dust-like stages, first order phase transitions and stages
of superheavy metastable particle dominance in the early Universe. The
mechanism of closed wall contraction can lead, in the inflationary Universe, to
a new approach to galaxy formation, involving primordial clouds of massive BHs
created around the intermediate mass or supermassive BH and playing the role of
galactic seeds.
