  The next generation of gravitational-wave experiments, such as Einstein
Telescope, Cosmic Explorer and LISA, will test the primordial black hole
scenario. We provide a forecast for the minimum testable value of the abundance
of primordial black holes as a function of their masses for both the
unclustered and clustered spatial distributions at formation. In particular, we
show that these instruments may test abundances, relative to the dark matter,
as low as $10^{-10}$.
