Primordial black holes (PBHs) are considered as a potential source of dark matter, despite the lack of direct evidence for their existence. PBHs smaller than about $10^{15}$ g would have evaporated by now, leaving interesting cosmological consequences. However, there are constraints on the mass ranges in which PBHs could contribute significantly to dark matter. These constraints are derived from various observations and theoretical considerations, including evaporation, lensing, dynamical effects, accretion, cosmic microwave background distortions, and gravitational waves. The main constraints come from the extragalactic $\gamma$-ray background, microlensing of $\gamma$-ray bursts, and observations of the Galactic center. The paper discusses the formation scenarios of PBHs, such as primordial inhomogeneities, scale-invariant fluctuations, and critical collapse, and explores the possibility of multi-spike mass functions. It also examines the implications of PBHs in resolving cosmological puzzles, such as the generation of large-scale structure and the formation of supermassive black holes in galactic nuclei. The paper concludes by discussing the unified PBH scenario and the potential for PBHs to provide a significant fraction of dark matter, even if they do not explain all of it.Primordial black holes (PBHs) are considered as a potential source of dark matter, despite the lack of direct evidence for their existence. PBHs smaller than about $10^{15}$ g would have evaporated by now, leaving interesting cosmological consequences. However, there are constraints on the mass ranges in which PBHs could contribute significantly to dark matter. These constraints are derived from various observations and theoretical considerations, including evaporation, lensing, dynamical effects, accretion, cosmic microwave background distortions, and gravitational waves. The main constraints come from the extragalactic $\gamma$-ray background, microlensing of $\gamma$-ray bursts, and observations of the Galactic center. The paper discusses the formation scenarios of PBHs, such as primordial inhomogeneities, scale-invariant fluctuations, and critical collapse, and explores the possibility of multi-spike mass functions. It also examines the implications of PBHs in resolving cosmological puzzles, such as the generation of large-scale structure and the formation of supermassive black holes in galactic nuclei. The paper concludes by discussing the unified PBH scenario and the potential for PBHs to provide a significant fraction of dark matter, even if they do not explain all of it.