  Most of the X-ray emission from luminous accreting black holes emerges from
within 20 gravitational radii. The effective emission radius is several times
smaller if the black hole is rapidly spinning. General Relativistic effects can
then be very important. Large spacetime curvature causes strong lightbending
and large gravitational redshifts. The hard X-ray, power-law-emitting corona
irradiates the accretion disc generating an X-ray reflection component. Atomic
features in the reflection spectrum allow gravitational redshifts to be
measured. Time delays between observed variations in the power-law and the
reflection spectrum (reverberation) enable the physical scale of the reflecting
region to be determined. The relative strength of the reflection and power-law
continuum depends on light bending. All of these observed effects enable the
immediate environment of the black hole where the effects of General Relativity
are on display to be probed and explored.
