  One of the Holy Grails of observational astronomy is to confirm the
prediction that black holes in the Universe are described by the Kerr solution
of Einstein's field equations of general relativity. This Topical Collection
provides a status report of theoretical and experimental progress towards
confirming the "Kerr paradigm" through X-ray astronomy, gravitational lensing,
stellar tidal disruption events, superradiance, and gravitational-wave
observations of black hole binary mergers.
