  We review the theory behind the formation of primordial black hole binaries
and their merger rates. We consider the binary formation in the early and late
Universe, emphasising the former as it gives the dominant contribution of the
present primordial black hole merger rate. The binaries formed in the early
Universe are highly eccentric and get easily disrupted by interactions with
other primordial black holes. We discuss in detail how the suppression of the
merger rate arising from such interactions can be estimated and how such
interactions lead to the formation of another, much harder, binary population
that contributes to the present merger rate if more than 10% of dark matter
consists of primordial black holes with a narrow mass distribution.
