  Over the last decades X-ray spectroscopy has proven to be a powerful tool for
the estimation of black hole spin and several other key parameters in dozens of
AGN and black hole X-ray binaries. In this White Paper, we discuss the
observational and theoretical challenges expected in the exploration,
discovery, and study of astrophysical black holes in the next decade. We focus
on the case of accreting black holes and their electromagnetic signatures, with
particular emphasis on the measurement of the relativistic reflection component
in their X-ray spectra.
