  Black holes are a common feature of the Universe. They are observed as
stellar mass black holes spread throughout galaxies and as supermassive objects
in their centres. Observations of stars orbiting close to the centre of our
Galaxy provide detailed clear evidence for the presence of a 4 million Solar
mass black hole. Gas accreting onto distant supermassive black holes produces
the most luminous persistent sources of radiation observed, outshining galaxies
as quasars. The energy generated by such displays may even profoundly affect
the fate of a galaxy. We briefly review the history of black holes and
relativistic astrophysics before exploring the observational evidence for black
holes and reviewing current observations including black hole mass and spin. In
parallel we outline the general relativistic derivation of the physical
properties of black holes relevant to observation. Finally we speculate on
future observations and touch on black hole thermodynamics and the extraction
of energy from rotating black holes.
