  The origin and properties of black hole seeds that grow to produce the
detected population of supermassive black holes are unconstrained at present.
Despite the existence of several potentially feasible channels for the
production of initial seeds in the high redshift universe, since even actively
growing seeds are not directly observable at these epochs, discriminating
between models remains challenging. Several new observables that encapsulate
information about seeding have been proposed in recent years, and these offer
exciting prospects for truly unraveling the nature of black hole seeds in the
coming years. One of the key challenges for this task lies in the complexity of
the problem, the required disentangling of the confounding effects of accretion
physics and mergers, as mergers and accretion events over cosmic time stand to
erase these initial conditions. Nevertheless, some unique signatures of seeding
do survive and still exist in: local scaling relations between black holes and
their galaxy hosts at low-masses; in high-redshift luminosity functions of
accreting black holes; and in the total number and mass functions of
gravitational wave coalescence events from merging binary black holes. One of
the clearest discriminants for seed models are these high redshift
gravitational wave detections of mergers from space detectable in the
milliHertz range. These predicted event rates offer the most direct constraints
on the properties of initial black hole seeds. Improving our theoretical
understanding of black hole dynamics and accretion will also be pivotal in
constraining seeding models in combination with the wide range of
multi-messenger data.
