| Abstract: | Astrophysical black holes only have mass and spin. They can be highly luminous if they accrete matter from their surroundings such as a companion star or gas clouds accumulated in the centre of a galaxy. The intrinsic simplicity of black holes means that the accretion process remains similar across the mass scale, from about 5 to over 10 billion Solar masses. How a nearby stellar mass black hole system works informs us as to how a distant trillion Solar luminosity quasar operates. The energy and momentum output of such a powerful quasar can influence the evolution of its host galaxy. How the accretion process works is the main topic of this talk. A significant fraction of the power is emitted in the X-ray band and illuminates the immediate surroundings of the central black hole causing florescence and backscattered radiation, collectively called X-ray reflection. Observations of X-ray spectral line shifts then reveal the gravitational depth, velocity and innermost radius of the accretion flow which is indicative of the spin of the black hole. The spin determines the energy released per unit mass accreted and so is an important factor in the power released. I shall discuss X-ray relection and review results obtained from both stellar mass and supermassive black holes. |