  An accurate calculation of their abundance is crucial for numerous aspects of
cosmology related to primordial black holes (PBHs). For example, placing
constraints on the primordial power spectrum from constraints on the abundance
of PBHs (or vice-versa), calculating the mass function observable today, or
predicting the merger rate of (primordial) black holes observable by
gravitational wave observatories such as LIGO, Virgo and KAGRA.
  In this chapter, we will discuss the different methods used for the
calculation of the abundance of PBHs forming from large-amplitude cosmological
perturbations, assuming only a minimal understanding of modern cosmology.
Different parameters to describe cosmological perturbations will be considered
(including different choices for the window function), and it will be argued
that the compaction is typically the most appropriate choice. Different
methodologies for calculating the abundance and mass function are explained,
including \emph{Press-Schechter}-type and peaks theory approaches.
