Astronomy at the very edge of black holes

Accretion onto black holes can liberate a significant fraction of the
rest mass energy as radiation and/or relativistic jets. It powers both
X-ray binaries and active galaxies. Most of the power is dissipated in
the immediate vicinity of the central black hole. Irradiation of the
flow produces a characteristic reflection spectrum which, through
relativistic blurring, carries imprints of the effects of strong
gravity. X-ray observations probe the innermost regions of the
accretion flow via X-ray spectra and variability. I will review our
current understanding of the phenomena associated with the X-ray
emission in accreting black holes and discuss past and most recent
observational and theoretical results in the field.