  This paper is a biased review of primordial black hole (PBH) formation and
abundance estimation. We first review the three-zone model for PBH formation to
help an intuitive understanding of the PBH formation process. Then, for more
accurate analyses, we introduce necessary tools such as cosmological
long-wavelength solutions, the definition of the mass and compaction function
in a spherically symmetric spacetime and peak theory. Combining all these
tools, we calculate the PBH mass spectrum for the case of the monochromatic
curvature power spectrum as a demonstration.
