  The existence of non-zero neutrino masses points to the likely existence of
multiple SM neutral fermions. When such states are heavy enough that they
cannot be produced in oscillations, they are referred to as Heavy Neutral
Leptons (HNLs). In this white paper we discuss the present experimental status
of HNLs including colliders, beta decay, accelerators, as well as astrophysical
and cosmological impacts. We discuss the importance of continuing to search for
HNLs, and its potential impact on our understanding on key fundamental
questions, and additionally we outline the future prospects for next-generation
future experiments or upcoming accelerator run scenarios.
