  In this paper, we study the potential of future colliders to explore the
parameter space of heavy neutral leptons (HNLs) through the dipole portal. We
consider hadron colliders such as the LHC in the high luminosity phase and
FCC-hh, and lepton colliders, such as FCC-ee. We consider various signatures
for the HNLs, including the missing energy signature and displaced decays, and
discuss the complementarity between the hadron and lepton colliders. In
particular, we find that thanks to a much clearer environment, FCC-ee may
search for the HNLs with masses up to $\simeq 30\text{ GeV}$ and proper
lifetimes $c\tau_{N}\gtrsim 1\text{ cm}$, which is well beyond the reach of the
experiments to be launched in the next decade.
